referring to fig1 - 5 , a plastic bag is shown generally at 10 . the plastic bag 10 includes coextensive front and rear panels 12 and 14 , respectively ( broadly , first and second panels ). the panels 12 , 14 are joined at the bottom 16 and sides 18 of the bag 10 . the bag 10 can be formed by extruding a tubular film ( see fig2 ), and the bottom of the tubular film sealed together to form the bag . alternatively , the panels 12 , 14 can be formed as a single sheet of material that is folded at the bottom of the bag 10 along a linear fold , and the sides edges joined together by fusion lines . the panels 12 , 14 can be joined in other manners within the scope of this invention . for example , the panels may be formed as separate sheets and fused together along the bottom and side edges . whether the panels are formed as a tubular film , formed as one piece and folded over , or formed as two pieces , the edges are considered to be “ joined ” together for purposes of this description . furthermore , in some embodiments , the panels 12 , 14 may not be coextensive . for example , the top of one of the panels can extend above the top of the other panel . the bag 10 can be any suitable size for a particular application . for example , the bag 10 can be a narrow profile produce bag having a width in a range of about 12 inches to about 14 inches and a length in a range of about 18 inches to about 24 inches . alternatively , the bag 10 can be a regular profile produce bag having a width in a range of about 10 inches to about 12 inches and a length in a range of about 15 inches to about 20 inches . the bag 10 can also be a t - shirt type bag having front and rear gusset panels and a cutout forming a pair of bag handles , the t - shirt type bag having a width in a range of about 7 . 5 inches to about 8 inches , a gusset in a range of about 5 inches to about 6 . 5 inches , and a length in a range of about 19 . 5 inches to about 24 inches . these examples are for illustration purposes only , as the bag 10 can be any size and configuration within the scope of the present invention . the panels 12 , 14 each have an outer surface 20 and an inner surface 22 . the inner surfaces 22 of the joined panels 12 , 14 define a bag interior 24 for receiving items placed in the bag 10 . the top edges 26 , 28 of the panels 12 , 14 are unsecured along their length to define an opening 30 permitting access to the bag interior 24 and its contents . the top edge 26 of the panel 12 is illustrated in fig1 as being curled away from the top edge 28 of the panel 14 , as will be described in more detail below . the bag 10 includes a grip to facilitate movement of the panels 12 , 14 with respect to each other for opening the bag . the grip can be any structure that permits gripping of the panels 12 , 14 to open the bag 10 , such as a co - extrusion of a sticky resin . in the illustrated embodiment , the grip comprises a sticky or tacky grip strip 32 . the grip strip 32 is a vertical strip that extends between the top edges 26 , 28 of the panels 12 , 14 and the bottom 16 of the bag 10 . the strip 32 is relatively tacky or sticky as compared to the remainder of the bag 10 , which is relatively smooth in comparison . the relatively smooth remainder of the bag 10 has a relatively low cof as compared to the grip strip 32 , which has a higher cof to improve grip . as illustrated , at least one of the panels 12 , 14 includes at least one strip 32 on the outer surface 20 of the panel . each of the panels 12 , 14 can include any desired number of grip strips . for example , each panel 12 , 14 can include one strip 32 ( see fig1 ), two grip strips ( see fig5 ), three grip strips ( see fig2 ), or any other suitable number of grip strips . each strip 32 preferably has a width w 1 in a range of about 0 . 25 inches to about 5 inches , such as from about 0 . 5 inches to about 2 . 5 inches . if the panel includes multiple strips 32 , the strips can be spaced from each other by a distance d 1 in a range of about 1 inch to about 10 inches , such as from about 2 inches to about 5 inches . in the illustrated embodiment , the first panel 12 has the same number of grip strips 32 as the second panel 14 , and each strip on the first panel is aligned with a strip on the second panel . however , other configurations are within the scope of the present invention . for example , the panels 12 , 14 can have different numbers of strips 32 , the strips on the panels can be staggered from each other , or only one of the panels can include a strip or strips . as illustrated , each grip strip 32 extends along the entire height of the panels 12 , 14 from the top edges 26 , 28 to the bottom 16 of the bag 10 . however , the vertical strip 32 may extend over only a portion of the height of the panels 12 , 14 within the scope of the present invention . preferably , the grip strip 32 extends to the top edges 26 , 28 and is immediately adjacent the opening 30 . as seen in fig3 , each grip strip 32 extends partially through a thickness t 1 of each panel 12 , 14 . the grip strip 32 has a thickness t 2 extending from the outer surface 20 toward the inner surface 22 of each panel 12 , 14 . in one embodiment , the thickness t 1 is in a range of about 5 μm to about 20 μm , and the thickness t 2 of the grip strip 32 is in a range of about 30 % to about 90 % of t 1 , such as between about 45 % and about 75 % of t 1 . in one embodiment , t 1 is about 8 μm and t 2 is about 5 μm . in the embodiment illustrated in fig6 - 8 , grip strip ( s ) 132 extend horizontally across each panel 12 , 14 between the sides 18 of the bag 10 . fig6 is a front elevation illustrating a portion of the front panel of the plastic bag as curled away from the rear panel for easy opening of the bag . the grip strip 132 is identical in material to the grip strip 32 described above . the grip strip 132 is relatively tacky or sticky as compared to the remainder of the bag 10 , which is relatively smooth in comparison . the relatively smooth remainder of the bag 10 has a relatively low cof as compared to the grip strip 132 , which has a higher cof to improve grip . as illustrated , at least one of the panels 12 , 14 includes at least one grip strip 132 on the outer surface 20 of the panel . each of the panels 12 , 14 can include any desired number of grip strips . for example , each panel 12 , 14 can include one grip strip 132 ( see fig6 and 7 ), two strips ( see fig1 ), three strips , or any other suitable number of grip strips . each strip 132 preferably has a width w 2 in a range of about 0 . 25 inches to about 5 inches , such as from about 0 . 5 inches to about 2 . 5 inches . if the panel includes multiple strips 132 , the strips can be spaced from each other by a distance d 2 in a range of about 1 inch to about 10 inches , such as from about 2 inches to about 5 inches . in the illustrated embodiment , the first panel 12 has the same number of grip strips 132 as the second panel 14 , and each strip on the first panel is aligned with a strip on the second panel . however , other configurations are within the scope of the present invention . for example , the panels 12 , 14 can have different numbers of grip strips 132 , the grip strips on the panels can be staggered from each other , or only one of the panels can include a grip strip . as illustrated , each horizontal grip strip 132 extends along the entire width of the panels 12 , 14 between the sides 18 of the bag 10 , from left edge to right edge . however , the horizontal grip strip 132 may extend over only a portion of the width of the panels 12 , 14 within the scope of the present invention . preferably , the grip strip 132 extends to the top edges 26 , 28 and is immediately adjacent the opening 30 . in one preferred embodiment , the edge of the grip strip is coterminous with the edge of the bag and the edge that defines the opening . as seen in fig8 , each grip strip 132 extends partially through the thickness t 1 of each panel 12 , 14 . the grip strip 132 has a thickness t 2 extending from the outer surface 20 toward the inner surface 22 of each panel 12 , 14 . in one embodiment , the thickness t 1 is in a range of about 5 μm to about 20 μm , and the thickness t 2 of the grip strip 132 is in a range of about 30 % to about 90 % of t 1 , such as between about 45 % and about 75 % of t 1 . in one embodiment , t 1 is about 8 μm and t 2 is about 5 μm . the plastic bag 10 is preferably made from a thermoplastic material comprising ethylene - based polymers , such as high - density polyethylene ( hdpe ), although other suitable materials are within the scope of the present invention . as noted , the plastic bag 10 includes at least one relatively tacky strip 32 , 132 and a relatively smooth surface of the remainder of the bag , the relatively smooth surface having a relatively low cof . the grip strip 32 , 132 preferably is made of a material with a higher coefficient of friction than the remainder of the bag . the grip strip is preferably made from a polyethylene of lower density than the hdpe used to make the remainder of the bag ; for example the grip strip comprises or is made from a material such as low density polyethylene ( ldpe ), linear low density polyethylene ( lldpe ), ethylene copolymers [ such as ethylene vinyl acetate copolymer ], or metallocene polyethylene or ethylene copolymer . examples of suitable metallocene lower density polyethylenes or ethylene copolymer include vistamaxx sold by exxon mobil , affinity sold by the dow chemical company , and engage sold by the dow chemical company . for example , in one embodiment the smooth portions of the bag are made from hdpe film having a density of greater than about 0 . 935 g / cm 3 , and the grip strips 32 , 132 are made from low density polyethylene having a density below about 0 . 935 g / cm 3 . using polyethylenes having different densities provide portions of varying coefficients of friction ( cof ). typically , the cof of the smooth portions is less than about 0 . 3 or from about 0 . 1 to about 0 . 3 ( e . g ., about 0 . 1 ). the cof of the grip strip ( s ) is at least about 0 . 5 or from about 0 . 5 to about 1 . 0 . these varying coefficients of friction can also be manifest by the relative cofs of the inner and outer surfaces . for example , typically the ratio of the cof of the grip strips to the smooth portions is at least 1 : 1 , more typically at least about 1 . 5 : 1 ( e . g ., about 2 : 1 ). as an alternative to using polyethylenes having different densities , the relatively smooth surface and relatively tacky surface can be imparted by incorporating one or more surface modification agents into the film used to prepare the plastic bag 10 . suitable surface modification agents for this purpose include slip additives and anti - block additives for the smooth surface and tackifiers for the tacky grip strip . in this manner , the relatively smooth portions and relatively tacky portions of the films used to construct the plastic bag are modified by virtue of the presence of one or more components ( i . e ., are chemically modified ). however , using polyethylenes having different material properties as described is preferred . the grip strips 32 , 132 are co - extruded with the panels 12 , 14 . the grip strips are made of a material ( i . e ., ethylene - based polymer having a lower density , as described above ) which is different from the material of the panels 12 , 14 , which is essentially embedded in the material for the panels 12 , 14 for co - axial extrusion . the process includes extruding two or more materials through a single die with two or more orifices arranged so that the extrudates merge and weld together into a laminar structure before chilling . co - extrusion can be employed in film blowing . the advantage of co - extrusion is that each ply of the laminate imparts a desired characteristic property , such as stiffness , heat - sealability , impermeability or resistance to some environment , all of which properties would be impossible to attain with any single material . the grip strips 32 , 132 permit easy opening of the plastic bag 10 . a user can grasp the bag 10 on the strips 32 , 132 and use the strips to move the first and second panels 12 , 14 relative to each other to open the bag . the user can easily grip the outer surface 20 of each panel 12 , 14 at the grip strip 32 , 132 and twist ( or move the panels in opposing directions ) to open the bag . the grip strips 32 , 132 provide a surface the user can easily grip to create relative movement between the panels , as opposed to the smooth surface of the remainder of the bag that is not easily gripped . furthermore , because of the material differences between the smooth surfaces of the remainder of the bag and the grip strip 32 , 132 in one embodiment of the invention , the top edges 26 , 28 of the panels curl outward away from each other to facilitate opening of the bag 10 . in particular , the hdpe film used to form the major panels of the bag is of higher crystallinity and has a faster crystallization speed than the lower density polyethylene material used to make the grip strip segment . for example , in one embodiment the hdpe film used to form the major panels of the bag preferably has a degree of crystallinity of at least 70 %, and the lower density polyethylene material used to make the grip strip preferably has a degree of crystallinity of less than 70 %. in one embodiment , the hdpe film has a degree of crystallinity in a range of about 70 - 80 % and the lower density polyethylene grip strip has a degree of crystallinity of less than 60 %, for example 45 - 55 %. thus , shortly after the film blowing co - extrusion process , the hdpe film reaches its equilibrium state after which the dimension of the hdpe film does not change . since the lower density polyethylene used to form the grip strips 32 , 132 has a lower crystallinity and a slower crystallization speed , its dimensions continue to change — i . e ., it continues to shrink — even after the hdpe material to which it is bonded has reached equilibrium and ceased changing . this differential imparts internal stress to the composite film . when the top of the bag is liberated by cutting or tearing at the bag opening , the internal stress relaxes and , as seen in fig4 and 9 , the shrinking of the grip strips 32 , 132 causes the top edges 26 , 28 of the bag 10 to curl outward away from each other . the degree of crystallization depends on the material components . for example , in one aspect it depends on the alpha - olefin content ( i . e ., short - chain branch ). for hdpe , butene - alpha olefin is used for copolymerization and its alpha - olefin content is low which results in a high degree of crystallization . lldpe has higher content of alpha - olefin ; so the degree of crystallization is lower than hdpe . the crystallization temperature ( peak ) is about 8 ˜ 10 ° c . below melt temperature . for example , with a 125 ° c . melt temperature for hexene - lldpe , the crystallization temperature is about 117 ˜ 115 ° c . in addition , one skilled in the art well understands that processing conditions affect the the degree of crystallization and crystallization temperature . for pure hdpe , the % of crystallization is about 70 - 80 %. in contrast , the degree of crystallization for pure ldpe is about 45 - 55 %. lldpe has a degree of crystallization between these two . and blending different types of resin such as lldpe / ldpe or lldpe / hdpe can be used to control the degree of crystallization . as a general proposition , the crystallization rate is too fast to be quantified for polyethylene . however , the invention in one aspect can be described and then recognized in terms of relative rates of crystallization without actually quantifying the rates . this relieves the assessor of having to be concerned about under what conditions — e . g ., from what temperature to what temperature , in what atmosphere , etc .— the rates of crystallization are to be measured . rather , one skilled in the art will understand that in this aspect the invention involves a co - extruded grip strip material which has a crystallization speed which is different from , preferably slower than , a crystallization speed of the panel material ; and the relevant conditions are those conditions under which the co - extrusion occurs and under which the bag is cooled , and the those conditions are not narrowly critical to this embodiment of the invention , provided the above - described conditions of internal stress are imparted . as seen in fig4 and 9 , multiple bags 10 can be formed by a blown film co - extrusion process and then sealed at 34 and perforated at 36 . to use a bag , a user tears the film at the perforation 36 to remove a single bag from the remainder of the bags formed together . upon tearing the film at the perforation 36 , the top edges 26 , 28 of the single bag 10 are no longer connected , and the internal stresses within the opposite panels 12 , 14 no longer cancel each other out . thus , upon tearing the film to remove a single bag , the panels 12 , 14 will begin to curl outward from each other due to the stress relaxation in the grip strips 32 , 132 . the user can then easily identify the opening 30 of the bag 10 and can easily open the bag . as seen in the embodiment illustrated in fig1 - 14 , multiple bags 10 can be formed by a blown film co - extrusion process , sealed and perforated as described above , and rolled together to form a bag roll 40 . as illustrated , each bag 10 in the bag roll 40 includes a vertical grip strip 32 on the front panel 12 and a vertical grip strip 32 on the rear panel aligned with the grip strip on the front panel . however , it is understood that the bags in the bag roll can have a different number or configuration of grip strips , such as in the embodiments described above . as seen in fig1 , a first bag 10 a in the bag roll 40 includes a vertical grip strip 32 a on the front panel 12 a and a vertical grip strip 32 a ′ on the back panel 14 a . likewise , as seen in fig1 and 14 , a second bag 10 b in the bag roll 40 includes a grip strip 32 b on the front panel 12 b and a grip strip 32 b ′ on the back panel 14 b , and a third bag 10 c includes a grip strip 32 c on the front panel 12 c and a grip strip 32 c ′ on the back panel 14 c . when multiple bags 10 a , 10 b , 10 c are rolled to form the bag roll 40 , the grip strips 32 of each bag align with each other . as seen in fig1 , when bag 10 a is pulled outward and upward to tear the bag at the perforation 36 , an upward force is imparted on the next bag 10 b in the bag roll 40 . this upward force causes the front panel 12 b of the bag 10 b to lift upward away from the back panel 14 b of the bag . the back panel 14 b of the bag 10 b does not lift upward because of the alignment of the grip strip 32 b ′ with the grip strip 32 c on the front panel 12 c of the third bag 10 c . fig1 illustrates the alignment of the grip strips 32 b ′ and 32 c on the bags 10 b and 10 c . the grip strip 32 b ′ on the back panel 14 b of the outer bag 10 b overlies and engages the grip strip 32 c on a front panel 12 c of the inner bag 10 c . the grip strips 32 b ′ and 32 c engage each other with a higher bond strength than the remainder of the panels . thus , because of the engagement of the grip strips , the panels 14 b , 12 c will remain stuck together even as an upward force is applied to the bag 10 b upon the removal of bag 10 a , whereas the front panel 12 b is separated from the back panel 14 b . removal of the bag 10 a therefore causes the next bag 10 b to open as the front panel 12 b of the next bag is lifted upward and the rear panel 14 b remains in place against the remainder of the bag roll 40 . in other embodiments , each bag 10 may only include one grip strip 32 , or the grip strips of overlying bags may not align with each other . the grip strip 32 of one bag will have a higher bond strength even with the smooth portion of another bag relative to the bond strength of two smooth portions together . thus , even a bag with only one grip strip on the surface facing the remainder of the bag roll would be opened by removal of a preceding bag . the grip strips 32 , 132 therefore permit easy opening of the bag 10 in multiple ways . a user can easily grip the grip strips 32 , 132 to move the panels 12 , 14 relative to each other to open the bag 10 . the material differences in the smooth portions of the bag 10 and the grip strips 32 , 132 cause the panels 12 , 14 to curl outward away from each other so that the user can easily identify the opening 30 of the bag and easily open the bag . and removal of one bag 10 from a bag roll 40 causes the front panel 12 of the next bag to lift upward while the grip strip 32 , 132 causes the back panel 14 of the next bag to remain in place , thereby allowing the user to easily identify the opening 30 of the bag and easily open the bag . the bag 10 can include different colored portions to provide information to the user about the location of the grip strips 32 , 132 . in one embodiment , the grip strip 32 , 132 is a different color than the panels 12 , 14 . this displays to a user the location of the grip strip 32 , 132 , which indicates where the user should grasp the bag 10 to open the bag . having described the invention in detail , it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims . when introducing elements of the present invention or the preferred embodiments ( s ) thereof , the articles “ a ”, “ an ”, the and “ said ” are intended to mean that there are one or more of the elements . the terms “ comprising ”, “ including ” and “ having ” are intended to be inclusive and mean that there may be additional elements other than the listed elements . in view of the above , it will be seen that the several objects of the invention are achieved and other advantageous results attained . as various changes could be made in the above products without departing from the scope of the invention , it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense .