The following figure uses matchsticks to create 5 small squares. The challenge is to create a figure that has only 4 small squares by moving just 2 of the matches. Every match is used and no matches are doubled-up, the squares that are formed are the same size as the original ones.
One is tempted to just discard two of the matches, however this is not a valid solution.
Another temptation is to double-up two of the edges. Essentially, this is equivalent to discarding 2 matches, so it is not allowed.
A clever (but wrong) solution involves making a figure with three small squares and one larger square.
If after giving it a good go you're feeling frustrated, you may want to look at this hint.