Batman's best friend Superman is a allergic to green chunks of his home planet. If Batman wants to find all of the Kryptonite on earth and destroy it or lock it away to keep Superman safe, how much should he look for?
Batman doesn't like the Big-Blue-Boyscout who hasn't even figured out he can fly yet. So Batman's going to collect all the Kryptonite in the world to line the Batcave with thus making it impossible to Superman to attack Batman's child kidnapping and government over-throwing operations. How much Kryptonite is available?
This Fermi problem was posed by the book Guesstimation.