Densaugeo is a hex that causes teeth to elongate at an alarming rate.
From the Story
Discovered in Book 4, Chapter 18, The Weighing of the Wands
"Hermione is hit by Densaugeo when it is cast by Draco Malfoy. Her two front teeth grow very fast past her bottom lip. Her teeth have grown down past her collar by the time she leaves to go to the Hospital Wing."