There are 12 people and a seesaw on an island, and you have the following information:
1) There is one islander who weighs differently than the other 11, but you don't know which one, nor if they are heavier or lighter.
2) You may use the seesaw to compare the weights of any two sets of islanders of your choosing.
3) Each of the twelve islanders is uniquely identifiable and can be weighed more than once.
What is the least number of times you need to use the seesaw in order to guarantee that you can identify the person who has the different weight?
Three weighings are sufficient!
This is just a variation of the old 12 balls and a scale balance problem.
See here for a solution:
http://www.curiouser.co.uk/puzzles/12bsolutions.htm
Solution (spoiler alert!)