Comment by Jun8

Here’s one nice such surprise to amaze any kid (or a date, if they’re of the right mold): get a ruler or a uniform stick, balance it on your index fingers by placing them randomly along the length. Then start moving your fingers together. They’ll always meet at the center (center of mass)! Your fingers take turns moving as if by magic. Your audience will first think that you’re purposefully moving them, so next let them do it.