Why is it Always About Poop?

I was checking the kids' backpacks after school one day and when I got to Finn's, I found this paper in his folder...

Apparently, he was learning about soil and was listing what soil has in it. What is the first thing he listed? Poop! Yes, poop! He is correct, but why is it always about poop with kids? At least he got a 100%, right?