When I was younger I used to spend a pretty strange amount of time reading through brain teaser books. As such, I occasionally hear a brain teaser and know the answer without being able to remember how in the world you get there. That's what happened with today's teaser.... I heard it on the Skeptics Guide to the Universe podcast this morning, answered immediately, then spent the rest of the train ride working out why I was right. I got there somewhere near Hyde Park.
Lets see how you do: A jeweler has 9 pearls, all identical shape and feel. He knows one weighs slightly more than the other 8, but all he has to measure with is a balance scale (one with two arms that compares weights to each other). What is the minimum number of times he needs to use the scale in order to figure out for certain which is the heavy pearl?