a paraprosdokian is a sentance where the second part changes the interpretation of the first part
examples (courtesy of wikipedia)
Groucho Marx- "she got her good looks from her father, he's a plastic surgeon"
Homer Simpson-"if I could say I few words, I'd be a better public speaker"
Mitch Hedberg- "I used to do drugs, I still do, but I used to also"
Bob Monkhouse - "I want to die like my father, quietly in his sleep- not screaming and terrified like his passengers"