It is called Elasticity.
Can air be pressed into a smaller bulk?
It can, by means of a weight.
When the weight is removed, what does the air do?
The air goes back to its former bulk.
Is air very elastic?
Yes; air is one of the most elastic substances known.
Why does a ball rebound when thrown against the floor?
Because the substance of the ball is pressed out of shape when it strikes the floor, and, in its effort to spring back to its former shape, it is forced to rebound.
Why does the ball rebound from the floor?