Inertia: Catch a Quarter

Place your hand by your ear and raise your elbow so your forearm is parallel with the floor.

Place a quarter on your elbow.

Quickly move your elbow down and attempt to catch the quarter in the same movement. 

Due to inertia, the quarter wants to remain in the same place, rather than move with your arm.  If you move quickly enough, you can catch the quarter before gravity pulls it out of your reach.