If one is a tooth and a whole set are teeth, Why shouldn't the plural of booth be called beeth?
Why is the verb in the clause a whole set are teeth plural? Doesn't Aspinea's answer apply here, that it should be singular [a whole set is teeth]?
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It's a linguistic humor and so, not all grammar rules shall apply. That's what I think. To throw a pun a little flexibility in syntax is allowed. That's why, maybe, this is like that.
I agree that a whole set is teeth makes the sentence more grammatical but on the other hand, if you think - If [one is tooth] and a [whole set] (now becoming plural as there are several toothS! [are teeth]. may make a sense though it's silly!
Another such with no strict rule is..
The masculine pronouns are he, his and him, but imagine the feminine she, shis, and shim!