What's the one thing most humans feel compelled to do when they see what appears to be a cute baby all snuggled up in a pram? They go over to coo at it and make silly baby noises. What they don't expect is the child's head to jump forth and for it not to be a normal child at all but, in fact, a devil baby.
As you can imagine, plenty of pants wetting and hilarity ensues. What a wonderful prank.