Where bones meet
A joint is any place where two bones meet. You have joints in your neck, shoulders, elbows, wrists, fingers, hips, knees, ankles, toes and lots more places.
Some joints move a lot, like your shoulder. Some move just a little, like the joints between your back bones. Some don't move at all, like the seams in your skull.
Try it: walk like a robot, without bending your knees or elbows. Hard, right? Joints make moving easy.
Six kinds of joints
Hinge joints bend one way, like a door. Your elbows, your knees and the middle joints of your fingers are hinges.
Ball and socket joints have a round ball that fits in a cup. They can swing almost any way. Your hips and shoulders are ball and socket joints.
Pivot joints let one bone turn around another. The top two bones in your neck make a pivot, so you can shake your head no.
Gliding joints let flat bones slide over each other, like the small bones in your wrist and ankle. A saddle joint, at the bottom of your thumb, rocks two ways, like a rider in a saddle.
Fixed joints don't move. The zig-zag seams between your skull bones are fixed joints.
Inside a moving joint
In a moving joint, the ends of the bones are covered with smooth, slippery cartilage. It's so slippery that the bones glide on it more easily than ice skates on ice.
The joint is wrapped in a little bag. Inside is a slippery liquid called synovial fluid, which you say like sih-NO-vee-ul. It works like oil in a squeaky door hinge.
Tough, stretchy straps called ligaments tie the bones together, so they don't slip apart. Tendons are different: they join muscles to bones.
Why knuckles crack
When someone cracks their knuckles, they stretch the joint. That makes room in the joint liquid, and a little gas bubble forms with a pop.
After a crack, you usually can't crack the same knuckle again right away. The joint needs a while before it's ready.
Myth: Cracking your knuckles gives you sore joints when you're old. Truth: Studies haven't found that it causes arthritis, a sickness that makes joints sore. Still, be gentle with your joints, and stop if it ever hurts.
Other joints can pop or click too, like your knees when you stand up. That's usually nothing to worry about.
The yes joint and the no joint
Your top neck bone is called the atlas. It holds up your skull, the way the giant Atlas in old Greek stories held up the sky.
When you nod yes, your skull rocks on the atlas. When you shake your head no, the atlas turns around a little peg on the bone below it, called the axis.
Try it: nod yes slowly, then shake no slowly. You're using two different joints!