This is hardly a new observation, but it's amazing how kids can go all day without dehydrating, but RIGHT AT BEDTIME (not 10 minutes before, not 5 minutes before, not 30 seconds before) it's like they've suddenly been in the desert for a week. You weren't thirsty at all at dinner (your cup is still sitting there half-full). You weren't thirsty for the last hour of running around/playing/reading/watching tv. You weren't even thirsty when you brushed your teeth (the water was right there!).
But the words "go get in bed" have some Pavlovian response.
It's uncanny.