A reader wrote in with this question: How come when I put salt on a melon, the melon seems sweeter? Then if I put more salt on, the melon turns salty?
Here's the answer:
The answer to the question lies in the nature of melons. Unless we look at a piece of melon under a microscope, we can't see that it's actually made up of many thousands of tiny cells. These cells have what are called "semi-permeable membranes".
That means that water can go into the cells or it can come back out again. For the purposes of today's discussion you can think of these cells as little water packets. Now one of the things that salt does is in effect to draw water toward itself.
If you have a region of high salt concentration next to one of low salt concentration, the water from the low-salt concentration area will move over to the high-salt area. By sprinkling a little salt on your melon you are creating a high-salt concentration area next to those cells.
Their water is drawn up to the surface where you bite, bringing flavor with it. The melon has become miraculously sweeter thanks to salt! Add too much, however, and the taste of the salt itself will overpower the effect, and it will taste salty.