Get this, they're so closely related that botanists created a plant that grows tomatoes above ground and potatoes below.

Potatoes have fruits as well - they look like little dark green tomatoes. Toxic of course, because nightshade.

There is more to a plant than just the fruit, you know. It just happens that the species (cultivar?) of nightshade that we grow for potatoes has tasty, starchy roots, while others have tasty, zesty fruits, and then one of them is eggplant.

Just be glad they didn't turn into crabs or cats.

... what? No, they're all nightshade plants. Not the same plant, mind, but still the same family.