There are plenty of things you can do in a lecture that don't involve actually having to listen to it. For instance, you can pretend that you're tickling the guy sitting behind you's nose—and then when he sneezes, fall about laughing at your magical powers.
That's what these two girls do in this hilariously subtle webcam prank, somehow managing such perfect timing that whenever one of them pretends to touch his nose, seconds later he sneezes. But, but how?
And the poor guy hasn't got a clue why they're snorting with laughter. All he can do is look over at them bemused, while dying a little inside.