Babies are born without kneecaps, having only cartilage in that area.
Why it is true
At birth, babies knees are made of cartilage, which facilitates childbirth and allows greater flexibility. This cartilage begins to ossify and turn into real bone only between two and six years of age.
Source: American Academy of Orthopaedic Surgeons
Would you rather be asked first?
This is the reading side of the site, so the answer is in the sentence above — that is the whole point of it. Test yourself works the other way round: the question comes on its own, and the fact and the explanation only appear once you have answered.