Pica is a disorder that makes people want to eat things that aren't food. If you have a strange craving for plaster or seashells, you may have pica.
While pica is sometimes a psychological disorder, it's often caused by a nutritional deficiency. A person who's getting much less iron (or some other necessary mineral) in their diet than they need might crave things like dirt or stones. Pica can be dangerous, since the objects sufferers eat can poison them, like paint chips containing lead, or cause a blockage in their intestines. In Latin, pica means "magpie," a bird that eats a wide variety of foods.