Use about 20 flashcards. Have a student say a number between one and twenty. Then start reading the flashcards. If the number is 7? for example, stop and do something special on the seventh card and on any multiple of seven. The special thing can be shouting, or not saying the card, etc. Do this in a group at first, but later go through the class one by one. Any student who does the wrong thing on the special number is out.