Black Magic?

In homeroom i have these kids that play a game and they call it the black magic game, I will give the points to who ever can tell me how they do it. One of them goes out and one stays in the room and when one of them is out of the room the other picks an objects in the room. When the person comes back, the person that has remained ask the the person who has just returned was it the clock or something like that several times before asking the correct object and sure enough the answer is a sure yes. PLEASE PLEASE PLEASE HELP ME!!!!!!!!!