Be specific.

We stopped by a friend's house today after dropping Jordan off at school and after picking beans, eating yogurt and chasing an unsuspecting pooch around the yard we ended up in the pool. I hadn't exactly planned to take Miles and Sedona (or myself for that matter) swimming today and was not adequately prepared.
So they skinny-dipped and I borrowed a suit. There was plenty of swimming fun and afterwards I realized that I had driven Jeremy's car not my own today. (Ask me about transmissions. . . I think they're swell). Hardly seems a problem, right?
Yeah, well, very much in the over-packed mom manner, my car is stocked with diapers, wipes, snacks, books, clothes, medicine, sunscreen - you name it. Jeremy's car, however, is not.

So there stood two wet, naked and chilly 2 yr olds without a dry diaper between them.

After a no-peeing-in-the-car pep talk, away we went (by this time it was time to pick Jordan up).
Well, we made it to his school ( I called him out 10 minutes early with two dry kids in tow, not wanting to press my luck any further. I saw how much pool water was consumed.)

All the way Sedona kept reminding me, "Dry, Mommy. No pee pee. Dry."

Then she peed in the driveway. The millisecond she disembarked.

So I guess the moral of the story is. . . . be specific. If you don't want her to pee until she is in a diaper or on a toilet - say that. Don't leave it at "don't pee in the car".