It is believed that by tapping on a watermelon and listening to the feedback sound, one can deduce whether the watermelon is ripe and tasty or not. This belief seems to be very common in some areas of the world.
Some references which mention it:
Some say a ripe watermelon will produce a hollow sound, while a “thick” or “solid” sound indicates a watermelon that’s not ripe or too ripe. Others say a hollow or “tight” sound is bad, and your watermelon should instead sound “firm.” Still others say a ripe watermelon should produce a B-flat sound. (What?!)
Unlike other watermelon suppliers who use the “tapping” method of quality control, Scotlynn uses the most advanced x-ray machines available.