Here's a fun challenge ( and an easy one) that is great to ask people at a party.
The setup is the following: you take 6 glasses and line them up in a row. Then, fill the first 3 with water as in the following picture:
|six glass problem|
The problem is this: You need to change the order of the glasses. What we want is the first glass to be filled, the second to be empty, the third to be filled, the fourth to be empty, the fifth to be filled, and sixth to be empty.
However, there is a restriction: you are only allowed to move one of the glasses.
Can you figure out how to do it?