Because your skin keeps germs out of your body, any cut, scrape or scratch that breaks it is a wound, and the first rule at every age is a short one: wash it, and tell a grown-up. Hands should be washed with soap and water before a cut is cleaned, and the cut itself is rinsed with clean water to wash the dirt away. Washing it with mild soap and water soon after it happens helps stop infection, while a bandage covers a small cut to keep it clean while it heals. Even small cuts and scrapes need to be cleaned and covered so they do not get infected.

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. Within the first few minutes of an injury, platelets in the blood begin to stick to the injured site, and fibrin then forms a mesh that acts like glue and binds the platelets to each other. Platelets are tiny parts of blood that help form the clot that works like a natural bandage. Hemostasis comes first, when vessels narrow and a clot forms, which stops the loss of blood and gives cells a framework to move across.

Platelets at an injury release chemical signals that attract more platelets, and then immune cells such as neutrophils arrive to clear bacteria and dead material from the wound. Neutrophils release proteases that break down damaged tissue, and since they live only about two days at a wound, they die and are engulfed and broken down by macrophages. In the first few days a cut can be swollen, red and sore, which is the immune system protecting the wound, and this stage lasts about two to five days. A clear or pale yellow fluid that seeps from a healing wound helps clean it, but yellow or green pus under or around a scab is a sign that the cut is infected.

A scab is a hard crust that forms over a cut or scrape, and it is a dried clot. It forms within seconds or minutes after the skin is hurt, and it works like a little shield that helps keep germs out of the cut. New skin cells appear underneath the scab, and the scab stays in place until they do. Once the skin underneath has healed, the scab falls off by itself.

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, and a cut that is kept moist and covered may not form a scab at all. If a scab does form, leave it alone. A scab can itch while the skin under it heals, but scratching it can break the new skin, and picking a scab makes a cut heal more slowly.

Until collagen is deposited, a fibrin-fibronectin plug is the main structural support for a wound, and repair cells use that plug as a matrix to crawl across. About two or three days after a wound forms, fibroblasts begin to enter it, and by the end of the first week they are the main cells in the wound. Collagen is a tough, white protein fiber that the body makes to join the edges of a wound. New blood vessels are needed because fibroblasts and skin cells require oxygen and nutrients, and tissue where new vessels have grown often looks red because of all the tiny capillaries.

Skin cells called keratinocytes come from the wound edges and from hair follicles, sweat glands and oil glands. These cells crawl across a wound, but they can only travel over living tissue, so a deep wound must first be filled with granulation tissue. Under a scab, the body repairs broken blood vessels and grows new tissue over about three weeks. As a wound heals its edges pull inward, so the wound gets smaller, and small cuts heal faster than big, deep ones.

Fibroblasts make the collagen that forms a scar, which works like a biological Band-Aid for damaged skin. Collagen is made quickly at first, but enzymes called collagenases break it down at the same time. Soon after wounding, making outpaces breaking down, so collagen builds up, and later the two become equal, so there is no net gain. A scar forms because new tissue grows back differently from the original skin, and the mark it leaves is a scar only after the wound has healed.

In healthy skin, collagen fibers are woven like a basket. In a scar, fibroblasts lay the collagen down quickly in parallel lines, so the tissue is thicker, weaker and less flexible than the skin around it. Scar tissue has no hair follicles, sweat or oil glands, or nerves, so a scar cannot do everything that normal skin does. Most scars fade as time goes by, although a scar can take up to two years to fade.

A scar is a quick patch that covers a wound fast, and speedy healing was an evolutionary advantage. Humans and other mammals mostly patch wounds with scar tissue, while some lizards, salamanders and fish can regrow lost body parts. Young bodies heal faster than older ones, and eating healthy foods gives the body what it needs to heal a wound.

Some warning signs mean a grown-up should step in. Spreading redness, more pain, swelling or pus are signs of infection, and a grown-up should call a doctor. 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. Even then the first rule stays the same: wash it, and tell a grown-up.

Fetal skin can heal without a scar, and scientists first saw this in lambs in the early 1970s. African spiny mice, which can shed patches of skin to escape a predator, then regrow skin and hair follicles instead of a scar. Each of these two animals shows that a scar is not the only way a mammal's skin can heal.

Words to know

• hemostasis: stopping the leak
• fibroblasts: collagen-making cells
• collagen: tough white fiber
• follicles: hair roots
• evolutionary: long-term change