When you push the plunger of a bicycle pump, a force moves through a distance and you do work on the air inside. Doing work on a gas means energy is transferred to it, and this can make its temperature rise.
Here is why. The plunger moves towards the gas molecules. A molecule that hits the moving plunger rebounds with more speed than it had before, in the same way that a ball hit by a moving bat leaves faster. The molecules therefore gain kinetic energy. Their average kinetic energy increases, and this is a rise in the temperature of the gas.
You can feel this when you pump up a bicycle tyre quickly. The barrel of the pump, near the outlet, feels warm because the compressed air has become hotter. The energy that heated the air came from the work done by the person pushing the plunger.
