Lilly blew my mind on Saturday. After dinner, I asked if she needed to go potty (not a common question, as I usually don't bring it up). She said yes, we went upstairs, and she peed. A lot. Then I asked if she wanted her diaper back, or underwear; she picked the latter and selected a purple polka-dotted pair.
I took her to the potty about every twenty minutes, and she went every time. She asked for a playground, so I put a towel in her carseat, put the kids' potty in the van, and off we went. She took potty breaks there too. When we were getting ready for bed, she drank her milk, then told me she had to pee. It took me awhile to convince her to put her diaper on for bed. She really liked her underwear.
It turns out that their mom had just so happened to pull the little potty out of the closet that morning, and both kids had wanted to use it. I have no idea what happened yesterday, but today I offered Lilly underwear again and she wore some - though she never peed on the potty, and she was playing in a sprinkler so I have no idea if she ever had an accident. Either way, we're on our way!