Well, I think we've made it to potty-training success! Sam has gone to preschool for 2 weeks with his "big boy underpants" on without any accidents. He also hasn't had even a small accident in several days. He hasn't mastered nighttime or #2 yet, but it's clear that these things will come soon.
To celebrate, last Saturday we all went out to McDonald's for lunch and then to Child's Play Toy Store to pick up Sam's potty reward, a marble run.
Incidentally, ever since he's been on track with going to the potty a lot of his defiance has dissipated. It makes me wonder which came first -- did he decide to cooperate with potty training as well as other things in life, or did his success in potty training lead to feeling more grown-up, thus acting more grown-up? Any thoughts?