This demonstration can not only be used to impress your friends but shows inertia.

 

Inertia is the tendency for an object to stay a rest until a force acts upon it.

 

In this demo we are looking at the orange. So it wants to stay where it is until a force acts on it, in this case the force that makes it move is gravity. This makes it fall in the glass.

 

The reason the water is put in the glass is to increase its mass. This increases the moment of inertia of the glass so it is less likely to move.