When you pump up your bicycle tires or football, you are making use of compressed air.
Inside a bicycle pump is a small device called a valve.
A valve allows a gas or a liquid to pass through it in one direction only.
Attached to the tire is another valve.
When the plunger of the pump is pushed down, the valve attached to the tire opens and air is forced into the tire.
As the plunger is pulled back, the valve on the tire closes so that no air can escape.
At the same time, the valve on the end of the plunger allows more air into the pump.
Next time the plunger is pushed down, this air is forced into the tire.
If you place your thumb over the hole at the end of a bicycle pump and push in the pump handle, you will compress the air inside into a very small space.
It will become more and more difficult to push farther.