bruises

You know how a bruise changes color over time? That's your body fixing the bruise by breaking down and reabsorbing the blood, which causes the bruise to go through many colors of the rainbow before it eventually disappears. You can pretty much guess the age of a bruise just by looking at its color:
When you first get a bruise, it's kind of reddish as the blood appears under the skin.
Within 1 or 2 days, the hemoglobin (an iron-containing substance that carries oxygen) in the blood changes and your bruise turns bluish-purple or even blackish.
After 5 to 10 days, the bruise turns greenish or yellowish.
Then, after 10 or 14 days, it turns yellowish-brown or light brown.

Finally, after about 2 weeks, your bruise fades away.