Sharon Morris, age 9, of Williamsport, Pennsylvania, for her question:

Why does red blood look blue inside our veins?

Inside your body, your blood flows through a branching network of tubes and tunnels. Around and around it goes on trips to visit the lungs and the heart and back on another round trip again and again. On the way it does a lot of busy work to keep the body going. It gathers a load of fresh oxygen from the lungs and delivers it to the busy cells. And this work causes the blood to change its color.

Oxygen causes the blood to blush with rosy red. When it delivers its oxygen, its color changes to bluish purple. You can see this bluish blood in your veins, dimly through your skin. Now suppose you cut or wound the skin. Some of the blood oozes out into the air.

And in the air it grabs a fresh supply of oxygen and this oxygen turns it red. Blue blood from a wound grabs oxygen from the air so fast that it changes color at once. We do not get a chance to see the blue color it had inside your veins.