Inspired by Brett Manley
Suppose you have 8 watermelons that are exactly same in appearance. They are equal in weight except for one. It is heavier than the others. You want to find the heaviest one by using a two-pan balance, where each pan can only hold one watermelon.
In the worst case scenario, at least how many times do you have to use the two-pan balance?
Note: The watermelons cannot be cut (since you do not know how the weight is distributed). You have to weigh the whole watermelon.