Here’s the kicker: Researchers have found that whole watermelons stored outside the fridge in a cool, dark place (around 55 or 60 degrees) last much longer than watermelons stored in the fridge (around 41 degrees). So, if you’ve already got a whole watermelon in the fridge, you should keep it there. But if you’re storing it outside the fridge, put it in the fridge about three hours before you plan to eat it in order to cool it down to just the right temperature.