What makes Mouse Droppings such a good prank?

Mouse Droppings works because the droppings appear gradually. At first, the victim might not notice a few small black marks. But as minutes pass, more and more droppings accumulate until the screen looks like it has been invaded by digital mice. The victim will try wiping the screen, checking for dead pixels, and restarting programs -- none of which will help because Mouse Droppings just keeps going.