Official Rules! Step 1 - Pick your opponent. There are few game losses that are as difficult to bear as the loss of a thumb war, so pick your opponent wisely. While preying on those with younger, slenderer thumbs is tempting, winning a lopsided thumb war will invariably leave you feeling a bit empty. So, find a good thumb war adversary who will prove to be a surmountable challenge. Step 2 - Warm up the thumb. Like a fighter pilot checking the wings and kicking the tires before a sortie, you should give your thumb a little test-run before you make any declarations. Swirl it around, move it about and wiggle it in the air to make sure everything is in good working order. Step 3 - Approach your opponent and state your intentions. While you might be eager to unleash the thumb war "chant," this is not quite the time. Instead, walk up to your opponent and give him or her a serious, sideways look to show them you're serious. Then ask them if they're up for a challenge and make a few teasing statements about their possibly being afraid to lose. Step 4 - Grasp your thumb war opponent's hand, interlocking your cupped fingers. As you rotate your thumb from left to right on the top of your opponent's hand, your opponent should do the same in the opposite direction. At the same time, state the thumb war change as follows: "1, 2, 3, 4. I declare a thumb war!" After the last word, let the games begin. Source = http://www.ehow.com/how_2074581_declare-thumb-war.html