Pee around forever
Luke: [age 7] "The girl next door has a doll that you can fill up with water from a fake baby bottle, and then it pees. I wonder if it came with its own toilet. Because if it did, then you could run a water tube from the toilet back to the doll, and it could just keep peeing, with the pee going around and around forever. That would be cool. And gross."
Maybe it's both
Luke: [age 6] "So, I have a question. When you eat soup, does it come out of your body as pee, or poop? Because it's got the liquid part that would come out as pee, but it's also got the food parts that are more like poop, so I don't know. Maybe it's both. You can't tell."