the.com/milk teeth

means The first set of teeth that children grow, which later fall out and get replaced by permanent adult teeth.

from The name shows up across many languages (German Milchzähne, French dents de lait) and most likely comes from the fact that these teeth erupt while an infant is still nursing or newly weaned — teeth that arrive during the 'milk' stage of life. Some older folk explanations claimed the teeth were literally made from milk or linked to a mother's diet during nursing, but that's a myth; the name is about timing and infancy, not composition.