The zombie river-crossing puzzle is a slightly tougher take on a classic riddle involving a fox, a hen, and some grain. The rules are as follows: three zombies and three humans need to cross a river in a boat that can only carry two at a time. There can't be more zombies than humans at any point on either side of the river, or the zombies will eat the humans. How do you get everyone to the other side? There's more than one valid solution!