Bones give us the freedom to do the things we want to do. They help us to stand up straight, to run, to jump, and to play. That's why it's important for bones to stay strong and healthy our whole lives.

Many older people have bones that have become weak and can break easily. Osteoporosis is a condition in which bones are fragile, making them fracture or break much easier. Osteoporosis doesn't usually show up in bones until adulthood, but it can start during childhood.

You make the bone that must last your lifetime when you are between the ages of 10 and 18 years old. This bone is known as your "peak bone mass." To reach the best possible peak bone mass, you must get enough exercise and calcium. Bones are like a bank account. If you deposit lots of calcium and exercise when you are young, you will have strong bones later in life.

Just as exercising your muscles makes them grow bigger, exercising your bones makes them work harder, which helps make them strong and dense. The best type of exercise to build bone mass is "weightbearing" exercise. Jogging and jumping rope are examples of weightbearing exercise because your feet and legs must carry your body weight as you do them.

• Walking
• Climbing stairs
• Hiking
• Dancing
• Cross-country skiing
• Sports such as basketball, volleyball, soccer, field hockey and softball

The 1996 Surgeon General's Report on Physical Activity and Health recommends that everyone over age 2 should get at least 30 minutes of moderate physical exercise on most days of the week. If you are just beginning an exercise program, start gradually, with 10 or 15 minutes, and then begin to exercise with more intensity for longer periods of time.

All through your life, your body continually removes and replaces small amounts of calcium from bones. If more calcium is removed than is replaced, bones become weaker and have a greater chance of breaking. Getting calcium from other sources, like yogurt or milk, means your body doesn't have to take as much from your bones. It also helps store calcium in the bones for later in life.

Babies less than 3 years old need about 500 milligrams of calcium a day, about the amount in two cups of milk. Kids ages 4 through 8 need 800 milligrams of calcium a day, or about two cups of yogurt. Older kids, ages 9 through 18, need at least 1,300 milligrams of calcium a day. That's one cup of orange juice with calcium added, two cups of milk, and a cup of yogurt.

Other kinds of food have calcium in them, but dairy products have the highest amount. If you're allergic to dairy products, look for foods with calcium added, like orange juice or tofu. Spinach, salmon, and burritos also are good sources of calcium.

Vitamin D helps your body absorb more calcium. Milk and some other foods have vitamin D added to them. You also get vitamin D from sunlight, so exercising outside helps your bones in two ways!