To-may-toes, to-mah-toes...

Scientifically, a tomato is a fruit: nutritious plant tissue surrounding seeds, designed to be eaten by animals so the seeds might later be (ahem) deposited elsewhere surrounded by helpful fertilizer. But in cooking terms, a tomato is a vegetable because of its low sugar levels. If someone asks for a fruit salad, or a fruit sorbet, you would not include tomatoes, because you are in a culinary context. If someone in a science lab wants to study the effects of something on fruit, tomatoes would probably be fine, against because of the context, and you would not use them if the test called for a vegetable.

Just to confuse things further, in 1893, the Supreme Court ruled that a law regarding "vegetables" (and not fruit) included tomatoes in the meaning of the word (Nix v. Hedden if you were curious), not for scientific or culinary reasons but because the justices agreed that Congress had created the law in question to cover a category that included tomatoes. Later laws then made the same assumption about what tomatoes were. The law is funny that way: the intended purpose of a law can affect the very meaning of words in that context, and then that meaning can carry on to other law.

I believe this sums up why perfectly reasonable people can't seem to see eye to eye ( when it comes to Prop 8 and gay marriage. )