Where a bone comes into contact with another bone, this is known as a joint, and the ends of the bones are covered with ligaments and cartilage.

Ligaments are tough fibers, and are meant to be soft enough to allow the joint to move but stiff enough to hold the bones in place in case the joint is damaged (a tendon snaps, e.g.). They are white and slightly silvery.

Cartilage is spongy and flexible (the stuff that your nose and ears are made of), and is meant to both cushion the bones, so they don't slam into one another, and lubricate the joints, so that the bones slide back and forth easily. The vertebrae in your spinal column sit on top of one another but are separated by ligaments and cartilage. These ligaments are more elastic than most other ligaments.

Your vocal chords are another type of elastic ligament. Where a bone comes into contact with a muscle, the muscle is attached to the bone with tendons. Tendons are similar to ligaments in that they are composed of tough, flexible fibers but tendons are more elastic and stretchy.

When you sprain something, you've injured the ligaments and when you strain something, you've injured the tendons or muscles.

Sprains result when a joint is suddenly moved out of its normal position, so that the ligament tears or stretches. Basketball players often sprain their fingers when the ball pushes the finger back too far. Strains, or pulls, result when a muscle or tendon is overused, hit with too much force, or overloaded. When I played soccer, I would repeatedly strain (or pull) my quad muscle in the top of my thigh, because as a defender Id stand around most of the time and then suddenly have to sprint full speed. The sudden change was an overload to my quad muscle and the tendons that attached it to the bone.

The only immovable joints are the ones that join the plates in your skull. When humans are born, the skull plates are separated and, as we mature, they grow together and form a solid skull. The scars, or joints, that mark the old plates are called sutures.

The knees, toes and fingers are hinge joints. The other types of joints are: ball-and-socket (shoulder, hip), sliding (vertebrae) and pivot (neck).

Vitamin D is made in the skin. Skin is made up of epidermis and a dermis. The epidermis is on top and is composed of an outer layer of dead skin cells and an inner layer of living skin cells (these are the ones that make the Vitamin D and the pigment in your skin). The dermis sits below the epidermis and houses hair follicles, muscles to move the hairs and make goose bumps, sweat glands, sebaceous glands (they make sebum, or skin oil), fat, blood vessels and nerves.