A friend tells you that she has invented a motor that you just add a tiny bit of gasoline to once and then the motor works forever as the kinetic energy and potential energy just keep transforming back and forth indefinitely. Why might you be skeptical

Entropy increases over time

Explanation:

Whereas energy is conserved, applying the principle of conservation of energy, it is true that the kinetic energy will be transformed into potential energy and vice versa. This is ignoring minor losses. However, over time, entropy increases so addition of gasoline just once may not work over a long time.