Dancing Ping Pong Ball experiment

Why does this happen?
Glass has a natural frequency where it vibrates. When you rub your damp finger on the outside of the glass your finger sticks to the glass because of friction and then slips because of the water which forms a cushion that reduces friction. When this slip and stick happens in equal intervals it causes the glass to vibrate.

The vibration from the glass pushes the air molecules around the glass in equal intervals, causing compression waves to form and ultimately make the sound we hear. The ping pong ball dances because it gets pushed by the vibration of the glass each time the ball comes in contact with edge of the glass.

A glass can vibrates back and forth 440 times per second, which is too fast for the eye to see.

TIPS:
Wash your hands.
Try lightly rubbing your finger and then rub harder. See which creates a better vibration.
Try Different glasses (Wine glass seem to work best for me)
Try two fingers