Potty Training?

Seth has generally not been interested in potty training. Here or there we can get him on the potty - but nothing with any regularity. Nana suggested we try making a new effort when Seth turned 3. While making lunch today I casually mentioned to Seth that since he is 3 we should think about going on the potty, to let me know when he had to go and that I had fun Elmo underwear for him. Five minutes later, Seth said he had to go potty (this was a first), he went got his treat (chocolate of course) and then put on his underwear. Whoa! Where do we go from here? We had one pee accident before nap when we put back on a diaper. I have been trying to explain to him that there are steps to the process. First we wear underwear during the day and then for nap and nighttime. Did I just start potty training him by accident?

UPDATE: After his nap Seth was no longer interested in the whole underwear thing...