Your skin keeps germs out of your body, so a cut, scrape or scratch that breaks the skin is a wound, and a wound needs care. Wash your hands with soap and water before you touch it, then rinse the wound with clean water to wash the dirt away. After that, tell a grown-up.

Meanwhile your body starts its own repairs. 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 stages blend into each other instead of waiting in line, a little like a team where the cleaners are still working when the builders show up.

The leak stops first. Platelets are tiny parts of blood that gather at an injury and help form a clot that works like a natural bandage. After seconds or minutes the clot dries. A scab is a dried clot, and it helps keep germs out of the cut. You could think of it as a little shield, worn only while the skin under it is being fixed.

Next comes cleanup. In the first few days, about two to five, a cut can be swollen, red and sore. That is the immune system protecting the wound, so it is a sign of work and not always of trouble. White blood cells fight germs in the wound and begin to repair it, and a clear or pale yellow fluid that seeps from the wound helps clean it.

Then the building begins. Under the scab, the body repairs broken blood vessels and grows new tissue over about three weeks. New skin cells appear underneath the scab, and the scab stays in place until they do. Collagen, a tough, white protein fiber, joins the edges of the wound. As a wound heals, its edges pull inward, so the wound gets smaller day by day.

Because the skin under a scab is healing, the scab can itch, and scratching it can break the new skin. Picking a scab makes a cut heal more slowly, which is why leaving it alone is the best advice, even though it is hard to follow when it itches. When the skin underneath has healed, the scab falls off by itself.

Sometimes a mark stays behind. A scar is a mark that is left on the skin after a wound has healed. It forms because new tissue grows back differently from the original skin, and a scar is less flexible than the skin around it. A scar can take up to two years to fade, so patience is part of the story. Keep it clean, and give it time.

How long does a cut take to heal? Most small cuts start to heal within a few days, and most minor cuts are healed in a week or less. Small cuts heal faster than big, deep ones, so the size of the cut changes how long you wait for the scab to fall away.

Some cuts need more than home care. Deep, large or dirty cuts need more care than small clean ones, and a doctor may need to look at them. Soil or dirt that gets into a wound can cause tetanus, so after a dirty or rusty cut a person should check that their tetanus shot is up to date.

Know the warning signs of infection. Yellow or green pus under or around a scab means the cut is infected, and an infection can make a wound larger and slower to heal. Spreading redness, more pain or swelling are signs of infection too. When you see any of them, a grown-up should call a doctor.

Words to know

• platelets: blood parts
• collagen: tough fiber
• tetanus: dirt-borne germ illness
• flexible: able to bend
• infection: germ trouble