Our 3 yr old son has been successfully peeing in the potty for a few months with very few mistakes. However, he poops in his underwear. We have tried "the doctor said".... the potty bell (he has only been going every few days) so that hasn't seemed to work....we have made him clean his underwear in the toilet which he thought was fun. When he sits on the potty to try, we leave him alone. We have tried rewards as well as being stern. We are at a loss, please let us know what to do next.