A Human Bone
In order to provide your body with the strength you need to keep your shape, to move, to hold your weight, and to protect your organs, your bones need to be very strong. However, in order to allow you to move easily, to run and jump and to be active, your bones also need to be as light as possible, and as small as possible.
Very large fat bones would certainly serve the purpose of protecting your body and providing strength. However, if they were too large, movement would be awkward. Very small, thin bones would allow you to move, jump and play, but are much more likely to break.
The solution is an amazing biological mix of medium sized, very strong, yet lightweight bones. What makes your bones so light, yet also very strong? The secret is in the chemicals. Your bones are made out of several very hard, yet lightweight chemicals. In order to keep your bones healthy, you need to make sure your body gets plenty of these chemicals. The most important of these is calcium. One very important source of calcium is milk. Thus, drinking milk makes your bones stronger.