It involves taking small green and red stickers and covertly sticking them on members of the opposite (or indeed same) sex.
Red: No Deal
If ever caught the usual plan is to lie through your teeth.
Man 2: OK
Woman: "Why are you putting that sticker on me"
Man 2: "Erm... I thought i saw it fall off and was sticking it back on again"
*Man runs away*