By Gods plan. The primary or "baby teeth" will become lose due to root resorption caused by the erupting adult teeth. When they become lose encourage your child to "wiggle" the tooth to make it more lose. Some kids will not move the tooth and some primary teeth have to be removed by a dentist.
Dissolving iceberg. Like an iceberg, the part of the baby tooth that you see is only about 1/4 the total size of the tooth. The root(s) under the gum comprise the other 3/4 of the tooth. Permanent teeth start to form before your baby is born. Gradually the permanent teeth lengthen and move toward the surface. As that happens, the root of the baby tooth dissolves away. It's all genetically controlled.
Spontaneously . When eating , by pulling on it. Sometimes they wont fall causing overcrowding in thus case the dentist will have to pull out the baby teeth.