Very well known game, in which combatants link fingers and attempt to get the other person to say 'mercy' by damaging the other person's wrists as painfully as possible. More fun if you use the other person's hands to hit themselves mid-bout, BUT watch out for the one in 2,000 children whose wrists bend back painlessly to meet their arms.
These children can never lose at mercy, and even if they're too weak to get a mercy out of you, there's no dignity to be had from trying to hurt someone who is smiling in that sickening way that shit people have when they're doing the one thing they're good at.
Additionally, if you let go, that constitutes a 'mercy', so you're stuck - effectively holding hands - with this leering feebler, until he gets bored. And he won't get bored, because he's doing the one thing he's good at.
Just be careful, is all.

For a more painful mercy, stand back to back with your hands between your legs, grab your opponent's hands, and pull upwards.
Impotence and red piss are guaranteed for the loser. Emphasising your victory by jumping as high as you can is considered brutal, but necessary.
The only downside to the game was that it looked as though you back-bumming - see bumming "y".