As a guy who can't hear the difference between knocking on a ripe melon, knocking on an under-ripe melon and knocking on my own ass, a "ripeness" indicator sticker on fruit would be awesome. This sticker, invented by a University of Arizona professor, can display to shoppers whether a fruit or vegetable is ripe, under-ripe or over-ripe.
The RediRipe stickers change from white to blue as the fruit ripens by detecting ethelyne gas emitted during the ripening process. The color change is on a 24-48 hour delay, depending on how fast the fruit ripens and excretes the gas.
Not all fruits make enough gas, and there's still a lot of research to be done, but soon we'll all be reading stickers instead of knocking on melons.
Professor invents 'ripeness' sticker [Yahoo News]