A lot of people wonder where vegans get their calcium. The human body is very efficient at absorbing as much calcium as it needs, so getting enough dietary calcium is rarely a problem. Plant foods are a great source of calcium, and it's actually more easily absorbed than calcium from dairy products. For thousands of years, people didn't drink cow's milk at all and their bones fared just fine. In fact, countries where people eat more dairy products have much higher rates of osteoporosis. If you'd like to read more about calcium and what foods are the best sources, you can read This article from Physician's Committee for Responsible Medicine. Dr. Fuhrman also has a great article on calcium and osteoporosis. Here's the link (it's a little more scientific, but still very interesting).
*Some of the largest animals (elephants, giraffes, cows, etc.) are primarily vegan and they build huge massive strong bones simply out of the plants they eat. If greens can grow an elephant, I'm sure they can grow a human!
*photo from Dr. Mcdougall