The world around us is full of colors: the grass is green, the sky is blue. What a pity it will be if people can not see all the colors. But there are people who are blind to certain colors.

Some birds and animals' eyes have few or no cones, so they can not see colors. Cats and dogs cannot see colors as well as we can.

Some insects can even see X-rays. Scientists know that there are other colors around us which insects can see but which we cannot see.

Human color blindness is a strange thing to explain. In a single eye there are millions of very small things called "cones". They help us to see in bright light and to tell the difference between colors. There are also millions of "rods" but they are used for seeing when it is nearly dark. They show us shape but not the color.

Human beings have favorite colors. Blue is often popular because it is the color of the cool sky and sea. Green is the color which makes us think of grass, field and forest. Yellow is the color of the sun. On the other hand, red is the color of blood and fire. It makes some people think of accidents, danger and blood. Black is the color of the night. In the dark we cannot see what is around us, so we are sometimes afraid of the unknown and do not like black as a color.