We've all been fidgety until we satisfy our need to eat something sweet, salty or savory. Scientists have long explored the mystery of cravings, and certain theories were made in an attempt to explain it.

A past study conducted by researchers at the University of California at San Francisco suggests that food cravings are triggered by stress or anxiety, in which cravings for sugar and fat increase when under stress.

Dana Hunnes, Ph.D., MPH, RD, a senior dietitian at Ronald Reagan-UCLA Medical Center, has listed three theories that might explain the reason behind food cravings: low levels of serotonin (a calming hormone that triggers the desire for certain foods); the hypothalamic-pituitary-adrenal axis (HPA) call for a break from the stress; and the body merely signals some deficiencies of some nutrients.

"If we are missing some nutrient in our body, or are deficient, our body will naturally seek out, or crave, foods or other items that contain that missing nutrient," Hunnes told Yahoo Health.

The third theory is the most accepted theory by scientists. For instance, people with iron-deficiency anemia crave for chalk, clay, coal, pebbles, dirt or even ice. Lemons, on the other hand, satisfy the body's need for vitamin C and iron.

Jessica Setnick, MS, RD, CEDRD, a senior fellow at Remuda Ranch Eating Disorder Treatment Center, suggests that cravings could be hormonal too, based on his observation on pregnant women.

"The fact that this happens when women are pregnant is a clue to me that it is hormonally related," she told Yahoo Health.

Cravings are normal, but experts warn that they could also lead to obsessive compulsive behavior. It becomes disturbing when one starts craving for weird stuff such as paint or sand, which could harm one's health. Experts recommend immediate therapy to address obsessive cravings.

"Ones that become compulsive and can cause permanent damage to your health, like eating indigestible objects like metal, dirt, hair, or inhaling toxic chemicals, are concerning cravings," said Robert London, MD, a practicing psychiatrist for more than three decades and a National Columnist for Elsevier/Frontline, to Yahoo Health. "These are considered a medical condition."