Your forearm bones are long bones, and in a growing child, something inside them is building new bone right now. Bones are not finished objects, like the beams of a house, but working parts of a body that is still growing. The bones give the body its shape and hold it up, and the place where the growing happens is small, rubbery and easy to miss.

That place is called a growth plate. Inside a growth plate, cartilage cells multiply and grow in length, and then they change into hard, mineralized bone. Cartilage is a rubbery, flexible material, and the tip of your nose is made of it too. So a growth plate works like a factory line that starts with soft material and ends with hard bone. Growth plates are the places where children's bones make new bone, so that the bones can grow longer.

A long bone does not grow from its middle outward. It grows at each end, next to a growth plate, and most long bones have a plate near each end. Long bones are the ones that are longer than they are wide, such as the thighbone, the forearm bones and the bones of the hands. The growth plate also helps decide the future length and shape of the grown-up bone. When a person is fully grown, the plates harden into solid bone, and after that the bone no longer grows longer.

The bone itself is made of two things working together, because a framework of a protein called collagen gives it structure and a mineral called calcium phosphate makes that framework hard. Bone is also living tissue, which means that old bits of bone are removed and replaced with new bone all the time. During childhood, much more new bone is put down than is taken away, so the skeleton grows and the bones get denser. The bone you build as a kid helps your bones for the rest of your life, because the most bone you will ever have is built when you are young.

Calcium is the mineral that helps build strong bones. As of November 2026, kids 9 to 18 years old need about 1,300 mg of calcium a day, kids 4 to 8 need 1,000 mg, and kids 1 to 3 need 700 mg. Milk, yogurt and cheese are among the best food sources. Leafy green vegetables, such as broccoli, kale and collard greens, also have calcium. Vitamin D helps the body take in the calcium it gets from food.

Growth also depends on sleep, and sleep is not one single state. Sleep comes in stages, and light sleep is easy to wake from while deep sleep is hard to wake from. In deep sleep, the body releases hormones that kids need for growth and development. Babies spend more than half of their time asleep, because a baby's first months are a period of big growth. As of November 2026, kids 6 to 12 years old need 9 to 12 hours of sleep in every 24 hours.

Food and movement matter too, since a growing body needs both building material and a reason to use it. Protein is needed for growth, energy and repair of the body's tissues, and muscles are made of protein. Healthy protein foods include poultry, fish, beans, eggs and nuts. Running, jumping and playing make muscles and bones stronger. As of November 2026, kids 6 to 17 years old need at least 60 minutes of active play or exercise every day, and it does not have to come all at once. Walks, jump rope and playground time add up. Eating extra food or extra vitamins does not make a child grow taller.

Growth does not move at a perfectly steady rate. From about age 2, kids usually grow about 2 and a half inches (6 centimeters) taller each year, but that is an average, and kids grow in spurts, with faster stretches and slower stretches. Kids tend to grow a bit faster in the spring than in other seasons. Babies grow fast in their first year. On average a baby grows about 10 inches (25 centimeters) longer in its first year. Each child grows at their own pace.

How do doctors keep track of all this? They measure how kids grow and mark the numbers on a growth chart, so that growth can be followed over time. Babies are measured lying down, which is called recumbent length, and kids who can stand are measured with a stadiometer, a tall measuring device.

Growth charts show where a child's numbers fall compared with other kids of the same age, and percentile lines on the chart show this. Girls and boys have separate growth charts because they grow in different patterns and at different rates.

Genes have a large say in how a child grows, and children in a family often grow in similar patterns. Kids cannot change the genes that decide how tall they will be, but healthy eating habits and being active help them make the most of it. Each child is still growing at a pace of their own.

Words to know

• cartilage: rubbery body material
• collagen: bone framework protein
• calcium: bone-building mineral
• stadiometer: height measurer