A good cupcake is definitely worth some negotiation.
The little girl in the video above barely gets halfway finished with her burger and fries when she asks for dessert. Dad doesn't fall for it, though. When he asks if she's full she says yes. Then, when he asks if she wants a cupcake she also says yes (darn those mind games!) The two go back and forth about why she wants a cupcake until she finally says the magic word: "Pwease."
As the video's description states, "negotiating for a cupcake is hard work!" True, but still totally worth it.