Any cut, scrape or scratch that breaks the skin is a wound. Tell a grown-up as soon as it happens, so that someone can help you look after it. Wash your hands with soap and water first, and then wash the cut itself with mild soap and water soon after it happens, which helps stop infection. Even small cuts and scrapes need to be cleaned and covered so they do not get infected, and a bandage on a small cut keeps it clean while it heals.

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 first stage, called hemostasis, comes first, as vessels narrow and a clot forms, which stops the loss of blood and gives cells a framework to move across. Platelets are tiny parts of blood that gather at an injury and help form the clot that works like a natural bandage. Platelets also release chemical signals that attract more platelets, so the first cells on the scene call in the rest of the crew.

Immune cells then arrive to deal with germs and debris. Among them are neutrophils, which clear bacteria and dead material from the wound. For the next few days, about two to five, a cut can be swollen, red and sore, because the immune system is protecting the wound. During this cleanup, white blood cells fight germs in the wound and begin to repair it, while a clear or pale yellow fluid may seep out and help to clean the wound.

A scab is a dried clot, and it forms within seconds or minutes after the skin is hurt. The scab is a hard crust over the cut, and it works like a little shield that helps keep germs out of the cut, so the repairs underneath can go on while the wound stays protected. Under it, skin cells called keratinocytes close the wound from its outer edges while fibroblasts fill the wound from below with collagen, a tough, white protein fiber that joins the edges of the wound. The tissue that fills a healing wound in this way is called granulation tissue.

New blood vessels grow into the granulation tissue and bring oxygen, which is essential for healing. As the wound heals, its edges pull inward, so the wound gets smaller. Under a scab, this construction work goes on for about three weeks. New skin cells appear underneath the scab, and the scab stays in place until they do, and then the scab falls off by itself when the skin underneath has healed. Then comes remodeling, which starts once the wound has closed and can go on for a year or two, so a closed wound is not yet a finished one.

The patch that remains is called a scar, which is a mark left on the skin after a wound has healed. It forms because new tissue grows back differently from the original skin. Most scars fade as time goes by, although a scar can take up to two years to fade. Some scars go further than that, and a keloid is a thick, raised scar that can grow bigger than the wound that caused it.

A scab protects a wound, but it can also slow healing, because wounds with scabs take longer to heal than wounds kept moist and covered. Keeping a cut moist with petroleum jelly helps it heal faster than leaving it dry. Even so, leave the scab alone, because picking a scab makes a cut heal more slowly. Spreading redness, more pain, swelling or pus are signs of infection, and a grown-up should call a doctor, so tell a grown-up if a wound seems to get worse. An infection can make a wound larger and slower to heal.

Healing does not run at the same pace in everyone. Small cuts heal faster than big, deep ones, and aging slows the healing of wounds, so older adults heal more slowly than younger people. Diabetes can slow wound healing too. A wound that does not heal on schedule is called a chronic, or long-term, wound, and it is a reason to tell a grown-up or a doctor.

Scar tissue never gets back the full strength or flexibility of the skin it replaced, and it has no hair follicles, sweat or oil glands, or nerves, so a scar cannot do everything that normal skin does. Humans and other mammals mostly patch wounds with scar tissue. Some lizards, salamanders and fish can regrow lost body parts instead, which means the same injury can end very differently in different animals.

Words to know

• keratinocytes: wound-closing cells
• granulation: new filling tissue
• keloid: raised scar
• platelets: clotting blood parts
• remodeling: strengthening stage