About the Teeth of Sharks

The thing about a shark is ā€” teeth,
One row above, one row beneath.

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

Still closer ā€” here, Iā€™ ll hold your hat:
Has it a third row behind that?

Now look in and... Look out! Oh my,
Iā€™ ll never know now! Well, goodbye.