change of clothes

A few days ago Calvin came home from the park a little while before us. When I got home I noticed he had changed his clothes. It is very unusual for Calvin to care at all about his hygiene so I was surprised to see him changed.  I asked him why he was in new clothes and he said, "oh, just because." And I asked, "Did you have some kind of accident?"  Calvin replied super matter of factly as he continued with what he was doing "yes, wet fart."