Part of the fruit-buying experience is squeezing and poking around the heaping pile to find the just-right piece. But with the Brix scale, you can actually know the ripeness of fruit ahead of time. As Flavor Alchemy points out, the ratio of sugar to liquid in the fruit increases as it ripens, so if the Brix number is higher, the fruit is sweeter.
Where does this magical number come from? A refractometer, which is a telescope-looking contraption that uses a speck of the fruit's juice to measure the Brix. Different fruits have different Brix concentrations, but Flavor Alchemy has a chart with varying ripeness levels for fruits ranging from apple to watermelon. Have you noticed Brix numbers in your supermarket's produce section? [via Foodit]