Buying Watermelons

I think it's supposed to be that if they sound hollow they're better or something like that.

Alright, here's the secret so you all can know. My dad actually taught me the way one day: you treat it like a drum. The shell of the watermelon would be the skin of the drum, the fruit would be the emptiness of the drum. If the skin is thinner, the drum will sound more hollow. If it is thicker it will sound a lot less hollow. You want to go for the thinner shelled watermelons (the hollower sounding ones) because that means there is more fruit on the inside.