The thing about a shark is—teeth,
One row above, one row beneath.

Now take a close look. Do you find
It has another row behind?

Still closer—here, I'll hold your hat:
Has it a third row behind that?

Now look in and...Look out! Oh my,
I'll never know now! Well, goodbye.

John Ciardi

http://www.poemhunter.com/poem/about-the-teeth-of-sharks/