Why Do Eggs Turn into Chicks When they are Kept Warm?

When the hen sits on the eggs it warms them, and a chick starts to grow in each. In 21 days the chick will be big enough to break out of the shell by itself. The eggs that are sold in stores are not fertilized so they won't hatch into chicks even if they are kept warm. Chicken eggs that have been fertilized will hatch after 21 days of incubation. With air, moisture and the right temperature the eggs can be hatched in an incubator even without mother hen. After laying their eggs, birds keep them warm until the baby chicks are hatched. Sometimes the mother will do this all by herself, sometimes the father bird will do it and sometimes the two will take turns.