If you don't know about Whack-A-Monty then the chances are you've not been playing enough New Super Mario Bros because it seems that anyone who's gone near the game has left with a very wobbly wrist and a serious addiction.
Whack-A-Monty is one of the minigames available in New Super Mario Bros which features Monty Mole and Luigi popping up from holes in the grass.
Your objective is to squash all the moles that poke their heads out of the soil. The more you hit the more time you get to hit more, hence the very addictive nature of the game.
But if you hit Luigi's head, which looks dangerously similar to Monty because of his moustache, you lose time. The graphics and sounds are exactly as you'd expect from the Mario boys.