A Fish, A Duck, and A Frog

We're all in the car going who knows where and Phillip says, "A fish, a duck, and a frog go in the big pile of poop."
Knowing that I had to have heard him wrong, I say, "In the pool?"
He says, laughing, "Nooo, in the poop!"
At this point, Chris and I are really trying hard not to laugh, not wanting to encourage talk of poop outside of the bathroom. I hadn't heard this particular story before, or for that matter, any talk of piles of poop, so I asked him, "Where did you hear this?"
He answers, "In the potty."
Chris and I eye roll each other because that makes absolutely no sense and it also means we will never know where he got this from.
He then continues to chant, "A fish, a duck, and a frog go in the potty a big pile of poop!" several more times.
Where do they come up with these things and why do they always involve poop???

These animals seem to have recurring themes in Phillip's life. He sleeps with a frog and a duck and his favorite book is Duck in a Truck which features a frog and a duck! So, who knows, maybe he really did come up with this little story all on his own. If so, I'm certainly glad he's using his imagination, but really, does it have to involve poop?!?
See ya later alligator...