Poll: How should I get my revenge?

My sister had a friend spend the night last night and they put shaving cream on my face. I want to do something one step up. I've already got my sister worried because i said I wouldn't do anything. Do you have any ideas? PS her friend is staying another night, so I can do it while they're sleeping or not. PPS Do u know any mildly harmful chemical that will burn when it touches the skin that I can find around the house?