Slinky

The spring descends the stairs due to the gravity force. When we leave the spring on the stairs, the inertia of the upper extremity makes it overcome its dead-point and descend to the lower stair.

Here the principle of conservation of energy is in force, the one which states that the sum of the present energies (except friction, that is converted into heat) remains constant. When the spring descends, we deal with three kind of energy: kinetic, potential and elastic.

Slinky has been protected by a patent for many years.