I'm posting one puzzle, riddle, math, or statistical problem a day. Try to answer each one and post your answers in the comments section. I'll post the answer the next day. Even if you have the same answer as someone else, feel free to put up your answer, too!
A large city in the United States had a problem with thefts of light bulbs from its subway system. Thieves would unscrew the light bulbs, leading to cost and security issues. The engineer who was given this challenge could not alter the location of the light bulbs, and he had very little budget to work with, but he came up with a solution.