I don’t know about all of it, but a heavy watermelon is well watered and more likely to be sweet since it’s had the resources to put tons of energy into growing all season. The same principle works for finding good citrus, you want something that feels heavy for its size.
Honestly, the cat helps me on that front. And unlike a child, I can ethically just give her some drugs and turn the TV when I don’t feel like dealing with her right that minute.