The definition of inbound is something that moves inward. An example of inbound is the freeway that goes toward the city center. 1  traveling or going inward.  Basketball inbounds.
Mass is a measurement of how much matter is in an object. Mass is a combination of the total number of atoms, the density of the atoms, and the type of atoms in an object. How to Measure Mass.
(note the four planets are GAS giants). Explain that these pictures all show MATTER. Have children log on to http://chem4kids.com/files/matter_intro.html to find the definition of matter. Matter is anything that takes up space and has mass (weight).