It's time for a bike ride, but wait, the tire's flat.

As you fill the tire from your hand pump you might notice parts of the pump getting hot, maybe even too hot to touch. Why would the pump get hot with all that cool air flowing through it?

A very small part of the heat comes from the friction of the air flowing through the hose. Just as your hands get warm from the friction of rubbing them together, the pump's hose gets warm from the air flowing through it.

However, the bigger reason for heat is the fact that you 're compressing billions of air molecules into a smaller space.

Ordinarily, air molecules repel each other, which is why you have to work so hard to push them into a small tire. When you compress them, the molecules respond by moving faster. The increase in the speed of the air molecules colliding with the inside of the pump creates the heat that you can feel.

Even without compressing the air, the friction generates some heat. Try working the pump without attaching it to the bicycle tire. Parts of the pump may get a little warmer, but nowhere near as hot as the pump gets when the air is being compressed.

Diesel engines use the same principle to burn fuel. When the diesel fuel is compressed inside the cylinder it gets so hot that it explodes, turning the engine.