This lesson presents a classic game-show scenario: You pick one of three doors in hopes of winning a prize. The host opens one of the two remaining doors which reveals no prize, then asks if you want to stick with your original choice or switch to the other unopened door. Which strategy gives you the best chance of winning? Approaches to the solution run from guesses to experiments to computer simulations (links to applets are provided) to theoretical models. The lesson also includes links to appropriate Internet extensions. This lesson plan first appeared in the April 1991 issue of Mathematics Teacher. (author/sw)