The sign for "tooth" uses an "X" hand to (pretend to) "tap" on a tooth twice.
You don't need to actually tap on your tooth. (You might occasionally see
someone actually make contact with their tooth, but in general anything that
slows down signing tends to be dispensed with when signing at high speeds.)
The sign for "tooth" is also commonly used to mean "glass," "ceramic,"
"porcelain," "china," or even "cement."