I say this to the kids all the time. I usually call it Rule #1 - "no fun." It's become a sort of game, me blustering into the room and thinking I'm funny, the kids rolling their eyes. But there's something more.
I want children who choose what's right because that's what they choose, not because I say so. It takes a lifetime to develop that kind of wisdom, but humor is a good place to start. Irony forces us to read faces and vocal intonation to assess whether someone is telling the truth. This is a critical skill for any young man or woman, and even young children are capable of it. When I play the fool, the kids have a chance, in a lighthearted sort of way, to gauge whether I, an adult they know and love, am telling the truth or not. They are required to listen carefully and not get led astray by my language. Most importantly, they must check what I've said against their own internal values.
Nope, that's just Joe making a joke. Move on.
Plus, it's a lot of fun.