Funny Dirty Joke

A girl has a sleepover with 3 guys. Then her dad comes down and paints her vagina purple. He tells the boys,''I'm going to sleep. When I come back i'm going to check and see if any of you had sex with my daughter.'' An hour later, he wakes up and checks everybody. When he checks the first two boys their penises are purple so he knows they had sex with his daughter. When he checks the last guy his dick is clean. The dad says ''Good job.''
When the third boy opens his mouth to say thanks, his mouth is purple!