How to kill Batman

Yeah, he's the smartest dude on the planet and one of the best hand-to-hand combatants and tacticians, but he's still just a dude. He can die. And I want to know how to do it. A fast-acting poison or virus? Lots and lots of hand grenades? Sniper rifle? It seems to me it would need to be quick and something so highly unlikely that it may even be overlooked by a brain obsessed with contingencies.