Babies are born without knee caps — rather, they have knee-cartilage. All babies are born with soft cartilage tissue where their knee caps will be. Although all the tissue is in place, it doesn't become ossified, or hard, until they are years older. Girls usually develop their knee caps at the age of three, a year or two earlier than boys. The last bone to ossify in the human body is the pubic tubercle, part of the hips. It doesn't appear until a person is 18-20 years old.

More facts about the bones:

• By the age of 20, the average young person has acquired roughly 98% of his or her skeletal mass.


• Adult human bones account for 14% of the body's total weight.


• The human femur, or thigh bone, is stronger than concrete. It also happens to be hollow.

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