One person is the "answerer".
The rest of the class are the "guessers".
The answerer has a word in mind (writes it down) and the guessers have to guess the word. They can only ask "yes or no" questions and the answerer may only say "yes, no, or I don't know". The guessers have to guess the word in a maximum of 25 questions. If they guess it, they win. If they don't, the answerer wins.