Your skin keeps germs out of your body, so any cut, scrape or scratch that breaks the skin is a wound, and a wound needs care right away. Wash your hands with soap and water first. Then rinse the wound with clean water to wash the dirt away, and tell a grown-up before you do anything else.

Even small cuts and scrapes need to be cleaned and covered so they do not get infected. Washing a cut with mild soap and water soon after it happens helps stop infection, so the first minutes after an injury matter.

Inside the wound, the body starts its repairs. Platelets are tiny parts of blood that gather at an injury and help form the clot that works like a natural bandage. A scab is a dried clot that forms within seconds or minutes after the skin is hurt. It is like a little shield that helps keep germs out of the cut.

Healing is a sequence, not a single event. A wound heals in stages that overlap: the leak is stopped, germs and debris are cleared, new tissue is built, and the repair is strengthened. The cleanup stage lasts about two to five days, and the wound can be swollen, red and sore because the immune system is protecting the wound.

The cleanup has two main tools. White blood cells fight germs in the wound and begin to repair it, and a clear or pale yellow fluid that seeps from a healing wound helps clean it. Yellow or green pus under or around a scab is different, because it is a sign that the cut is infected.

Next the body builds. Under the scab, it repairs broken blood vessels and grows new tissue over about three weeks, and oxygen carried by new blood vessels is essential for healing. New skin cells appear underneath the scab, and the scab stays in place until they do. The edges of the wound pull inward, so the wound gets smaller.

To join the edges of the wound, the body makes collagen, a tough, white protein fiber. The new tissue grows back differently from the original skin, and that is why a scar forms. A scar is a mark that is left on the skin after a wound has healed.

A scar is less flexible than the skin around it. A scar can take up to two years to fade, though most scars fade as time goes by. A healed scar should be protected from the sun with sunscreen of SPF 30 or more.

Some things slow healing down. Picking a scab makes a cut heal more slowly, an infection can make a wound larger and slower to heal, and diabetes can slow wound healing. Young bodies also heal faster than older ones.

So when should a grown-up step in? Spreading redness, more pain, swelling or pus are signs of infection, and a grown-up should call a doctor. Wash it, tell a grown-up, and let the doctor decide the rest.

Words to know

• platelets: blood parts
• collagen: tough fiber
• wound: broken skin
• flexible: able to bend
• infection: germ trouble