Sometimes called Gigglebelly, Ha! Ha! Ha! is an exercise in the ridiculous.
On the floor or in the grass, have the kids lie down in any formation as long as every child has his or her head on another child's belly (the kids should form a chain, so that every child has another player's head resting on his or her belly, as well).
At the whistle, the first child you point to should say "Ha!" The one whose head is on that child's belly should then say "Ha! Ha!" the third should say "Ha! Ha! Ha!" and so on.
If you can make it all the way through the line before pandemonium results, you may indeed have a group of professional clowns in your midst.