There are many moments worthy of celebration in potty training. The first successful go. The first time they take themselves. The first dry day. The first dry night.
But the really critical moment--the one where you know it's going to take this time--is when they realize that they have latched on to the universal excuse.
Chores too tedious? I gotta go potty!
Bedtime too early? I gotta go potty!
Church service too long? Older siblings too pesky? Gotta go, gotta go, gotta go.
Smart children learn to ration this precious commodity so that the grownups cannot thwart this new scheme.