The jet car

To fill up the tank of this car it is enough to inflate the balloon. Now the car can zoom like a rocket!

The direction of the car movement is opposite to the direction in which the balloon deflates. We can treat the car as a rocket, the air as fuel which is removed with a certain speed. Each time a given mass of fuel is removed, the speed of the rocket increases.

The less air there is in the balloon, the smaller the speed of the deflating air, and the car slows down as a result of friction forces, and eventually stops. The same happens to a car when it runs out of petrol.