Once, when she was ... 6? ... and I was babysitting her, we were eating some watermelon. She asked me about the seeds and why they were there since they didn't taste good and you weren't supposed to eat them anyway. I told her that's how fruit grow more fruit. You plant the seeds and then later, you get a bush or a tree or something that those fruit grow on.
So, then she wanted to save the seeds for me and her to plant and grow a "watermelon tree"
I knew this was ridiculous, but what was the harm? She and I planted a bunch of watermelon seeds, to grow a watermelon tree, on the side of my house (at the time).
Of course, a watermelon tree never grew, but there is a ridiculously tall (taller than normal) Rose-of-Sharon tree at almost that exact spot at my mom and dad's house.