He reserves number two for the diaper
What's going on: There could be any number of reasons your kid will give up the diaper to pee but will simply refuse to poop in the potty. He may not want to take a break from the action to sit still on the potty, for one, or he may be fearful after toilet water unexpectedly splashed up during his last poop. The diaper might make him feel secure, so giving it up entirely is a bigger step than it seems. Often, constipation's to blame. It hurt last time, so why would he do it again?
Take it slow. Break the transition from Pull-Up to potty into baby steps. "If your child will only go in a diaper, have him do so in the bathroom, then progress to having him sit on the potty in his diaper," Pantley says. "Once he's used to this, suggest taking his diaper off and putting it into the potty-chair bowl as a 'pocket' to catch his poop."
Pump up the fiber. If constipation is the culprit, load your child up with high-fiber foods like beans, most fruits and vegetables, and whole-grain pastas and breads (skip the cheese and bananas, which can make constipation worse). If that doesn't solve the problem, have a talk with your child's doctor.
Try a little bit of everything. When Katie was balking at pooping, we decided to take a breather for a month. Then we put her on a doctor-recommended high-fiber diet, and gave her a sticker every time she sat on the potty. She soon discovered that pooping on the potty wasn't really so bad. It took a mishmash of techniques -- and a giant Costco box of lollipops -- but Katie's now out of diapers, and I've learned a few tricks to use on her sister next year.
Lisa Milbrand also writes for Modern Bride, Budget Travel, and her blog, Themamahood.com.