Despite the extravagant colors of the flowers bees visit, until the early 1900s it was commonly thought that bees were entirely color-blind. To test this, zoologist Karl von Frisch set out an array of dishes on cards. The dish on one card, the only blue card among varying grays, contained sugar water. Once bees learned to visit this card and dish combination, he varied its position in the matrix. Next, he replaced all the cards and dishes with a new set of identical materials, only now leaving the blue card's dish empty. The bees nonetheless returned to the blue card. If their vision were monochromatic, they would likely have found at least some of the gray cards indistinguishable from the blue.