Any cut, scrape or scratch that breaks the skin is a wound, so even a small wound deserves careful attention. Tell a grown-up when you get one. Hands should be washed with soap and water before a cut is cleaned. Then the cut itself should be washed 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.

The first stage of healing is called hemostasis, and it comes first: blood 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 at an injury also release chemical signals that attract more platelets, and then immune cells arrive to deal with germs and debris. Immune cells such as neutrophils clear bacteria and dead material from the wound, so the first crew of cells calls in the next one.

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 acts like a little shield that helps keep germs out. When the skin underneath has healed, the scab falls off by itself, and picking a scab makes a cut heal more slowly.

During the first few days a cut can be swollen, red and sore, and this is the immune system protecting the wound. That stage lasts about two to five days, while white blood cells fight germs in the wound and begin to repair it. A clear or pale yellow fluid that seeps from a healing wound helps clean it.

The building stage comes next. Skin cells called keratinocytes close the surface of the wound, while fibroblasts fill the wound from below with collagen, a tough, white protein fiber that the body makes to join the edges of a wound. The tissue that fills a healing wound is called granulation tissue, and new blood vessels grow into it to bring oxygen and nutrients.

Under a 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. The last stage, remodeling, starts once the wound has closed and can go on for a year or two. 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.

A scab protects a wound, but it can also slow healing. 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 is there, leave it alone: picking a scab makes a cut heal more slowly.

Healing speed is not the same for everyone, because young bodies heal faster than older ones. Most small cuts start to heal within a few days, and most minor cuts are healed in a week or less. Eating healthy foods also gives the body what it needs to heal a wound. So the speed of repair depends on the body and the wound.

Some signs mean a wound needs a doctor. Spreading redness, more pain, swelling or pus are signs of infection, and a grown-up should call a doctor. Yellow or green pus under or around a scab is one such sign, and black edges around a wound can be a sign of dead tissue that needs a doctor's attention.

Not every wound is a small, clean one. 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.

When the work is finished, a mark may remain. A scar is a mark that is left on the skin after a wound has healed, and it forms because new tissue grows back differently from the original skin. A scar is also less flexible than the skin around it. Most scars fade as time goes by, and a scar can take up to two years to fade.

Words to know

• hemostasis: stopping the leak
• platelets: clotting blood parts
• fibroblasts: collagen-making cells
• collagen: tough protein fiber
• remodeling: strengthening stage