An old chestnut
goes like this:
There is a square table with a small well at each corner, just the size to hold a glass. There is a glass in each well, some right-side-up and some upside-down.
Your challenge: while blindfolded, try to get all the glasses right-side-up or all of them upside-down. You will be told when you have succeeded. However, you may only touch two glasses at a time, and the table rotates randomly between the times you get to touch glasses.
How can you guarantee all four glasses are turned the same way within no more than 5 attempts to turn glasses?