The magician starts out displaying THREE differently sized glasses, which are lined up from smallest to largest and are all shown to be empty.

The smallest glass is picked up and filled with milk from a milk carton. The small glass is displayed, and the magician looks at the glass and thinks that he wants a bigger glass of milk.
Unfortunately, there is no more milk in the milk carton.
So instead, the magician pours the milk from the smallest glass into the next bigger glass in the line up. The milk fills the glass to the top!
The magician takes this glass and fills up the next biggest glass in the line up, and it also fills the glass to the top!

Easy to Perform!
You supply the liquid!