I originally purchased a potty seat when my son was around one year old. He had began to tell me when he had 
just pottied in his diaper so I thought maybe it was time. I was wrong. Then at eighteen months he began asking to sit on the potty so we went through the routine of taking off the pants and diaper to sit there over and over again with no results. After my daughter was born he regressed and didn't want anything to do with the potty.
Out of nowhere the other day he told me that he needed to poop so we sat on the potty. Again nothing. I grabbed a couple of cars hoping that since he mentioned poop but hadn't gone in his diaper, that there would be a good chance this time. He played with cars but still nothing.
I then played the "big boy card". I told him that big boys that go poop in the potty wear big boy underwear while revealing a pair of Thomas Train underwear. He was immediately intrigued and wanted to wear them but after putting them on he went straight to playing with trains!
End Result: We are just not ready yet!
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Tips for Potty Training in Public