"No dirty butts on the rug" rule

So, this evening, we were at my grandmother's house. At one point, my dad helped Phin go to the bathroom. After he was done, my dad came out and said, "I didn't know the 'no dirty butts on the rug rule' was in effect. Apparently when Phin was done peeing, he sat down on the rug in the bathroom to put his underwear on, then went "oh, I can't put my dirty butt on this rug, or Grandma won't let me come back to this house." Too funny, right? I must have said that to him at some point, but I sure can't remember it.