There are three lockers, each with two drawers. Each drawer contains one item - a pearl, or a clump of coal. In one locker both drawers contain pearls, in another both lockers contain clumps of coal. In the third locker one drawer contains a pearl, and the other contains a clump of coal.
You do not know which locker is which. You open one drawer and find a pearl. If you are allowed to open one more drawer, which one should you open to maximise your chances of finding another pearl?
Related puzzles: Game show: Three doors, one treasure