The informant is Chinese-American where both of her parents are from China, but she has never been there herself. She said that her mom had several superstitions that she would tell her as a child. The informant seemed to feel fondly about this habit of her mother’s.
We were on a group trip to Catalina Island and we were having a picnic on the beach. One of the things we were eating, as a group, was almost an entire watermelon. In the middle of our picnic, the informant shared this superstition. After she said it everyone laughed, and some said that they had heard something similar.
My mom used to tell me that you’re not supposed to eat watermelon seeds, because if you do that you– you’ll grow a watermelon in your stomach.
I would guess that the informant’s mother told this superstition to her daughter, in part, as a way to warn her against eating watermelon seeds. While it probably won’t make a watermelon grow in your stomach, it isn’t the best for you either. The informant seemed to perform the piece of folklore somewhat ironically as a way of sharing her fondness for her mother with her friends.