Normally, your hair gets its color from pigment, little colored granules that form inside the structure of the hair. Polar bear fur, on the other hand, has structural color, which comes from how light bounces around the structure of the hair itself. It’s this effect that makes a polar bear look white—or sometimes green.

Until recently, it was widely believed that a polar bear’s transparent guard hairs act like natural fiber-optic conductors, absorbing and funneling sunlight to their black skin underneath. But research now shows that this isn’t quite how it works. Instead, a very small percentage of UV light makes it down a hair shaft, and less than a trillionth of red or violet light does, too. The rest of the light scatters and reflects, producing the polar bear’s white appearance.

This is how polar bears blend in with their surroundings, and also helps them absorb the sun’s heat to keep themselves warm. But sometimes, zoo visitors have noticed that some polar bears seem to be a bit of a different shade—perhaps gray on cloudy days or orange at sunset. The reason for this is that, in some environments (like the concrete floors in zoos), tiny plants called algae can grow inside a polar bear’s hollow hairs. This tints the space where the hairs usually scatter sunlight to create their white coloring with a color that’s a little more like a polar bear’s environment.

This is a great experiment to try with your kids or students—and you may be surprised by the results! Try a few variations, such as a different type of food in the water, and see how it changes the appearance.