"Didn't wash his hands" alarm Arduino prank

I got an idea from a Farside cartoon and thought I'd pass it along. It was of a guy coming out of a bathroom with an alarm going off over the door saying "Didn't wash his hands!", I think it would make a pretty funny prank to pull on people and should be doable!

I figure it should be possible for a single occupancy bathroom, basically you could have the Arduino detect a person as they come in the door, and if it doesn't detect the sink being used before the person sets off the door sensor again it sounds an alarm, assuming since the sink wasn't used the person didn't wash there hands.

The door sensor is pretty easy though I'm not sure how you'd detect the sink being used (sensor hidden in the drain maybe?)

Just thought I'd throw the idea out there.

Edit: My apologies to the moderators, I just realized why they moved this thread. I completely misunderstood the purpose of the section I put it under, which was "Suggestion FOR THE Arduino Project", not "Suggestions FOR AN Arduino Project" like I thought. Oops sorry, I was at work, running out of time on my break and was rushing when I posted it.