Thirty white horses

Thirty white horses upon a red hill;

Now they tramp,

Now they champ,

now they stand still.

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This is another one of those riddles that you can think about, perceiving your own body in a new way. Technically, it should be 32, although that doesn't fit the meter as nicely! (And plenty of people have 28, which also doesn't do as well with the meter.) I'm not a big fan of the tramp/champ rhyme there: I think I'd prefer stomp and chomp!

Teeth.