This story from yesterday's New York Times is a fascinating look at the monstrous world of teratomas, which are tumors that can grow various organs and body parts including teeth. The word teratoma literally means monster tumor (you can see why below).
Frightening though they may be, teratomas are usually benign, meaning they don't spread. But they can cause major problems if they crop up in the wrong place, like in the brain, because they increase in size rapidly.
Elizabeth Svoboda explains (registration required) how some scientists believe teratomas are valuable tools for evaluating cancer drugs, and how one ethicist believes the tumors could solve the embryonic stem cell dilemma.
