Once a prisoner is called, he goes in and he can either turn the light on or off, it doesn't matter. Once he's done this, he'll kill himself. Since he can't be called again, the warden must call another person. That person will go in, and then kill himself. This will continue, for about 99 days, and on the 100th day the last person will enter and say, "all 100 have entered if you include me." Pretty gruesome, I know, but it works (I think).