Why do kittens have blue eyes?

Kittens are born with blue eyes due to the absence of melanin in their irises. Melanin is the pigment that gives color to the hair, skin, and eyes of humans and animals. In kittens, the melanocytes (cells that produce melanin) in their irises have not yet fully developed, and so their eyes appear blue because of the scattering of white light through the iris of the eye. Refracted (scattered) white light is the same effect that makes the sky blue.

Image: MikeB

As kittens grow older, their melanocytes become more active and start to produce melanin. This can cause a change in eye color from blue to green, yellow, or brown. The exact timing of this change in eye color can vary depending on the breed of the cat, but it typically occurs between 6 and 12 weeks of age.

It's worth noting that some cat breeds, such as Siamese and some other pointed breeds, have a genetic trait that causes them to retain their blue eye color into adulthood. However, for most cats, blue eyes are a temporary feature of kittenhood that will change as they mature.