a collapsed bag of unitary construction is made of supple polymeric material , having first and second opposed contiguous flat sidewalls joined at edges and at the bottom but free or with user - releasable weakening at upper edges . the upper edges define a bag mouth . appropriately situated graspable points at or near bag edges allow the sidewalls to be tensioned or snapped in opposing directions thereby causing the interlayer cling , to which supple polymeric bags are renownedly susceptible , to be readily disrupted for the full distance of contact between the tensioning points and thus for virtually the entirety of the bag mouth to be opened .