However, this one cracked me up hardcore, because I thought of doing it chem last year too.
"Today, while I was sitting in chemistry, a girl asked if she could have some of my water. After she drank it, I told her with mock horror that she drank dihydrogen monoxide (which is just water). She started screaming and told our teacher. He pretended to be shocked and sent her to the nurse. Then he gave me extra credit. I love chem. MLIA"
Go you! I was going to say "I need a geeky science icon" (which is true too), but then I realised that the act is totally the sort of science!geek prank that Rachel Carson would play in undergrad, like the time she put red food colouring and a 'hazardous material' sign on the bottle of alcohol that kept being stolen from. (And it worked.)