'Rhythm' is one of the longest words without a vowel.
Why it is true
Since the letter 'Y' often acts as a vowel in English, words like 'rhythm' contain no standard A, E, I, O, or U vowels.
Source: Linguistics
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This is the reading side of the site, so the answer is in the sentence above — that is the whole point of it. Test yourself works the other way round: the question comes on its own, and the fact and the explanation only appear once you have answered.