Students see a classic "magic trick" illustrating inertia. Once you understand the science behind the magic, it will be hard to pull the rug out from under your students!
The glass of water has inertia — it will not move unless a big enough force acts on it. The force pulling on the glass of water is the friction between the tablecloth and the glass. Since the tablecloth and glass are slippery, the friction is quite small — it's enough to pull the glass along slowly, but not enough to accelerate the glass quickly when you snap the cloth fast!