Parents are always fascinated by their baby's eye colour, not just because it's a genetic trait, but because it can change as their child grows.
But why does this happen? The answer has to do with melanin.
Melanin is a type of pigment that not only affects eye colour, but the colour of your skin and hair. According to Douglas Fredrick, a pediatric ophthalmologist at Stanford Children's Health in California, "The amount of melanin in the iris, the coloured part of the eye, determines what colour a person's eyes will be."
The more melanin you have in your eyes, the darker the colour. For instance, brown eyes have a lot of melanin and blue eyes have very little. Green or hazel eyes have a medium amount.
If a baby is born with light-coloured eyes, this can change if melanin levels increase over time. Mental Floss host Craig Benzine explains: "Newborns don't have the levels of melanin that they'll eventually have; the amount increases over time, which is why eyes often start blue then change to another colour."
This change usually happens when a baby is around six months, but the change can occur up until the age of three.
Watch the clip above for a more in-depth explanation.