Did I ever tell you about the time I convinced my younger sister that each person has a quota of talking, and once that's used up she has to be silent?

That's absolutely brilliant! Can I use that on my kids? Nah, they're too old (and too used to my jokes) to fall for it.

You know, when they were younger (like around 6 and 4), and we were on a long car ride or some other time when we had to be quiet, we'd play "The Silent Game" --- the first person to say anything loses. It worked quite well for almost a year, actually.