When you pick up the watermelon, it should feel 'heavy'. Basically, a ripe melon is going to feel heavy for its size. You want to surprise yourself. If a watermelon is light, it is not ripe.
Knock on the watermelon. A ripe watermelon should 'sound' hollow.
Finally, check the bottom of the watermelon. There will be a yellow spot on it. The smaller this yellow spot is, the riper the watermelon is going to be.
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