Rhyming Riddle 5

Here is today's riddle:

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Two in one, but one in ten,
and only one in two.
Can you tell me what they are,
or do you need a clue?

clue:

Three, seven, nine: they all have two.

clue:

There are also two in four and five and eight.

clue:

But there's only one in six.

reveal/hide answer

Vowels.

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There are all kinds of ways you can remix this riddle and the clues, based on one in two, six, and ten; two in one, three, four, five, seven, eight, and nine... and you can go farther: two in twelve and twenty; three in eleven, thirteen, fifteen, and sixteen; four in fourteen, seventeen, eighteen, and nineteen. Have fun!

More Rhyming Riddles with Clues.