Repetitive ingestion of nonnutritive matter at an age where, developmentally, one should already be able to distinguish between edible and inedible objects. Patients with pica have been known to ingest chalk, soap, paper, feces, soil, wire, string, wood, and many other substances. Generally if this eating pattern continues for a month or more, you can safely say that the person has pica. It is recommended that doctors test possible pica patients for low zinc and iron levels as it tends to occur in people with malnutrition.
Example: A child discovers some scaling lead paint in a closet. He keeps returning to the closet to peel and eat the sweet paint chips.
Here is a very interesting site that covers causes, symptoms and all sorts of other information about pica: http://www.nlm.nih.gov/medlineplus/ency/article/001538.htm