Penny in the shot glass trick

This guy shows us how to put a penny into two shot glasses stacked together filled with water without spilling any of the water. Since water has a lot of service tension, you can move the glasses just enough to insert the penny.

You'll notice that a little bit of water spills out of the glass. Well thats because the penny is replacing the space that the water occupied, so it has to go somewhere. And that is what caused the little spill at 0:18 that all of the people on YouTube are complaining about.