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In a biological sense, a fruit is the developed ovary of a plant (the vessel that holds the plant's eggs), and examining a fruit gives clues about its past struggles. Before humans, the red flesh of strawberries was a decoy for flyby nibbles from birds. Avocados appealed to elephant-like creatures called gomphotheres, which had intestines wide enough for the animals to swallow the fruit and excrete its hefty seed somewhere else. The day gomphotheres went extinct, thankfully no one told avocados. Nine thousand years passed before the Aztecs invented guacamole.
As for what constitutes a fruit in 2018, sweetness has little to do with it. Tomatoes are fruits, but so are eggplants, peppers, and olives. Peanuts and almonds and walnuts are fruits. So are parts of the world's six top crops---wheat, corn, rice, barley, sorghum, and soy. Oftentimes, things that masquerade as vegetables, like pea pods, are definitely fruits. Which is not to cast shade at vegetables; they are, by definition, almost fruits. To botanists, vegetables are any other edible part of the plant that doesn't contain seeds. Roots, such as carrots, potatoes, and parsnips, are vegetables. Lettuces are seedless, so they're vegetables, too, as is garlic.
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Daniel Stone (The Food Explorer: The True Adventures of the Globe-Trotting Botanist Who Transformed What America Eats)