ELI5: If iron is in our blood, why is blood red instead of metallic silver?

Blood is red instead of metallic silver because the iron inside it is chemically bonded with oxygen, just like reddish-brown rust on an old bicycle.

Think about everyday objects made of shiny, silver iron, like a clean metal nail or a brand-new playground swing. When you leave that silver nail outside in the rain, it reacts with the air and water, turning into reddish-brown rust. The iron did not disappear; it just changed its color because it combined with oxygen.

Inside your body, a very similar process happens:

• Special Backpacks: Your blood has millions of tiny red cells that carry a protein called hemoglobin. You can imagine hemoglobin as a little backpack holding a single, microscopic speck of iron.
• Catching the Air: When you breathe in, oxygen enters your lungs and snaps onto that tiny speck of iron inside the backpack.
• Changing Colors: Just like the rusty nail outside, when iron grabs hold of oxygen, it completely changes how it looks. It reflects bright red light to your eyes instead of looking like a shiny coin.

When your blood is full of fresh oxygen from your lungs, it turns bright, cherry red. Once it drops off that oxygen to your muscles and brain, it becomes a darker, berry-red color—but it never turns silver, because the iron is always working as a chemical partner rather than a solid piece of metal.