That was the question posed to me by my two-year-old daughter. She already knew examples of vegetables. She wanted to know what makes a vegetable a vegetable.
I didn’t know, and I later found that the answer has a few twists and turns. From Wikipedia’s Vegetable article:
The term “vegetable” generally means the edible parts of plants. The definition of the word is traditional rather than scientific, however. Therefore the usage is somewhat arbitrary and subjective, as it is determined by individual cultural customs of food selection and food preparation.
Generally speaking, a herbaceous plant or plant part which is regularly eaten as unsweetened or salted food by humans is considered to be a vegetable. Mushrooms, though belonging to the biological kingdom Fungi, are also generally considered to be vegetables, at least in the retail industry. Nuts, seeds, grains, herbs, spices and culinary fruits are usually not considered to be vegetables, even though all of them are edible parts of plants.
In general, vegetables are regarded by cooks as being suitable for savory or salted dishes, rather than sweet dishes, although there are many exceptions, such as pumpkin pie.
For a livelier version of the same topic, I liked “What is the difference between a fruit and vegetable?” from YES Mag, a Canadian youth science magazine:
The answer depends on your relationship with the two items. If you’re stocking the produce department at a grocery store, a tomato is a vegetable. If you’re a plant scientist—a botanist—a tomato is a fruit. Cucumbers, pumpkins, avocados, and peppers are all fruits. Culturally, however, the grocer is going to call them vegetables.
A fruit is the ripe ovary or ovaries of a flower—the mature ovary of a seed-bearing plant. Let’s say you’ve got a tomato plant with those little yellow flowers all ready. A bee comes along and fertilizes the flower. The flower starts developing into a fruit with the seed inside. (There are four kinds of fruits, which explains fruits such as pineapple and blueberries, but let’s not get into that.) And, hey, guess what? Nuts are fruits. True nuts that is, chestnut and filberts come to mind.
Vegetables, however, are the roots (eg, carrot), tubers (eg, potato), leaves (eg spinach), stems (eg, celery), and other bits of plants that you might eat. For a botanist, a vegetable is sort of like the umbrella word for all the edible parts of a plant. Just to keep life interesting, mushrooms aren’t plants at all, they are a kind of fungus.
Let’s just keep with the cultural distinctions!
I’m still working on how to explain this to a two-year-old.