Einstein is famous because of his thought experiments ("Gedankenexperimente"). For the foundation of General Relativity he designed the elevator thought experiment.

In the animation are sketched three different situations.

(1) a piece of weight of 1kg in an elevator at rest,

(2) the same object with a reduced weight in an elevator which is hold by a cable, and

(3) the object in a free falling elevator exhibiting a vanishing weight.

This equivalence principle based experiment is a corner stone for Einsteins General Theory of Relativity: Locally acceleration and gravitation are the same.