This is the most simple computer prank I do. It takes a total of 3 button clicks and you’re done. Have fun.
Walk up to your friends computer, doesn’t matter if they are using it right now. Press Ctrl+AlT+ Any arrow key other than Up.
If their graphics card allows it, it should rotate their monitor in the direction you pressed. The key here is doing it fast. He or she wont see what you pressed. Now simply walk away.