After their daughter was born, the linguist Guy Deutscher and his wife decided to try an experiment. The new parents taught little Alma all the colors of the rainbow and beyond, in the way most parents teach most kids those colors: they pointed to objects—an apple, a grape, a stretch of bright grass—and told their daughter the color of those objects. Red, purple, green.
The pair made one omission in this prismatic approach to their daughter's education, though: Deutscher and his wife didn't tell Alma the color of the sky. They wanted to see what would happen to a developing mind when that mind had not been informed about one of the oldest truisms we have: that the sky is blue.
One day, as father and daughter were taking a walk, Deutscher pointed to the sky and asked his daughter what color it was.
As Radiolab tells it: Alma was confused. To her, it seemed, the sky was not a thing; therefore, it could have no color. A little more time went by. The pair went for a walk. Deutscher asked again. This time, Alma answered that the sky is white. A little more time went by. It wasn't until much later that, when asked of the color of the sky, Alma responded with the answer most of us would consider to be obvious.