This is one of my favorite puzzles so far. You are given 8 blocks that must be placed in a box. They will fit and the box will be "totally filled." Once you are done, slide the lid on the top to complete the puzzle. Your test subjects might feel pretty smart by knowing that they solved this puzzle within a few minutes. However, you have a surprise. Pull the extra piece from the lid and tell them to place it in the box. I've tested this puzzle with 20 people so far and the common response is "No Way!". But yep, it can be done.
This video shows the solution but doesn't answer the why. Do you see a difference?