When I was little, I knew that "wee-wee" was stored in the bladder. But for some reason, I thought that "B-M" was stored in the buttocks. Explained why your butt tended to stick out a bit. Of course, it never occurred to me to check for any change in size after "unloading."
A childhood belief about weeing and pooing

others believed things like this
- I used to think that poop was stored in your buttocks and that's why they're squishy.Anon
- I used to believe pee was stored in your stomach and poo was stored in your back.— Rob
- i used to think that your butt cheeks is where your poop was stored. Anon
- I knew that dairy farms have silos so I figured that they're where the milk is stored.
— marianne - When I was little I thought that my urine was stored in my scrotum until I needed to wee.— Bryan
- I believed that all of my blood was stored in the calves of my legs. — Julia