I've got one of those. Point it at your skin, and it tells you if you're alive or not. It counts blood pulses. Under a tenner on ebay.
I'm designing another. It determines whether you're a HighwayMan character or a JohnD, Nosenowt, character.
You stick it in your gob, and drop your strides. If you're an HWM, sure enough the light is shining out for all to see.
For the other, you, er, sit on it. You've heard of Black Light? Well this is Brown LIght. You make the mistake of opening your mouth and it pours out. It gets everywhere.