The Elevator Experiment

Stand on a bathroom scale inside an elevator. When the elevator accelerates upward, the scale reading jumps — you temporarily "weigh" more. When it accelerates downward, the reading drops.

This is exactly what Einstein thought about in his famous thought experiment. A person in a sealed box cannot tell whether they are standing on Earth or accelerating upward in space at 9.8 m/s². Gravity and acceleration produce identical effects.