Before we tell you, let's go over a few things...
What you know: Your bones get weaker as you get older.
What you probably don't know: Before they get weaker, they get stronger: Up to 90 percent of peak bone mass is acquired by age 18 in girls (age 20 in boys), and bone mass keeps growing until around 30. Once you hit menopause, that's when you start losing bone mass—rapidly at first, and then at a slower rate. So seizing the opportunity to build your bones during peak growing season (i.e., your 20s) fortifies them against fractures, breaks and a stooped posture, says Sabrina Strickland, MD, an orthopaedic surgeon at New York's Hospital for Special Surgery. If you're over 30, it's not too late: you can still protect your already-fully-grown bones against thinning.
What you know: Osteoporosis is pretty common among women over 50.
What you probably don't know: Though about 16 percent of women over 50 are diagnosed with osteoporosis, many more women that age (approximately one in two!) will break a wrist, a hip or some other part of their skeleton because of weak bone mass.
What you know: Exercise builds bone mass.
What you probably don't know: The best bone workouts meet certain criteria, says Wendy Kohrt, PhD, professor of medicine at University of Colorado at Denver, who studies how exercise affects bone density. They must be high-intensity enough to generate a maximum stress on the skeleton, dynamic enough to generate the signal to the cells to grow more bones and put unique demands on the skeleton that requires it to adapt. So swimming and cycling, while good for your heart and muscles, aren't very effective when it comes to bone-building. Soccer, in which you're sprinting down the field, pivoting and lunging to chase an opponent, and jumping to score a goal, is amazing (it's one of Kohrt's favorites). Runners-up include field hockey, tennis, and hiking with a pack.
What you know: Weights are good.
What you probably don't know: Kettlebells can be even better. You can use them to perform moves that meet all of Korht's bone-building criteria.
Here's one terrific example