What has a mouth full of teeth in a perfectly straight row, but never bites anything?

A zipper.

Did You Know?

The riddle plays on the word "teeth" — the interlocking metal or plastic bits on a zipper are literally called teeth, lined up in a perfect row like a jaw. The zipper was invented in 1893 by Whitcomb Judson, but it wasn't called a "zipper" until 1923, when B.F. Goodrich named it after the "zip" sound it made.