Can you bend your knees? 
 
D. Leela and classmates, Chennai 
 
We did a nice project on joints in school. Actually, it was Nivedita who found the rubber cap of the medicine bottle and was playing with it. I asked her what it was. "Nothing," she said. "It's just the rubber cap that seals in the medicine in some bottles. It has such a nice, soft feel that I like to hold it!" 
 
Of course it got passed around after that, everyone touching and stroking it. Then someone suggested Kho-kho and everyone ran off to play, forgetting the rubber cap. The game ended abruptly when Manisha hurt her knee and came back limping. For a while, she kept bending and stretching her leg, trying to estimate the damage. Then she gave it a good stretch and decided it was fine. We all went home. 
 
When we came back, we found Maya playing with the cap. As soon as she saw us, she hid it rather suspiciously.  "What have you got there," asked Navya at once. "Nothing," said Maya and put her hand behind her back. After some persuasion, she showed us what she had done. 
 
She had taken a metal hair clip and pierced the cap with it on one side. She was moving the clip up and down, up and down. Manisha giggled, "That looks just like what I was doing to my knee!" I got an idea. I found another clip and put it in on the other side. Then I held the whole thing up by the clips and kept moving them up and down. 
 
It really looked like a knee that was bending and stretching with the rubber cap like the knee cap in the centre! "Hey," said Nithya, "the cap is perfect. That's exactly how we learned the knee cap works: it prevents the legs from bending the other way." 
 
We showed it to Kamala-akka at school the next day. She really liked it. "It's so nice to think of a science toy made of every-day materials," she said. We all gaped: we never thought of that! 

Then Kamala-akka told us more about joints. Just think about it: if you didn't have joints in your body, you couldn't move! Forget about walking and running.  
 
Many bones need to be stiff so that they protect the soft parts inside. For example the skull protects the soft brain and the ribs protect all those important organs in our chest. 
 
But some bones such as in our hands and legs must move, otherwise we won't! They move because of the way bones join together at a joint. Of course, muscles do the actual work in pulling and pushing the joint. And muscles won't work without energy, which means food.  
 
Coming back to joints. Bones are joined to other bones by tough fibres called ligaments. You may have heard of some athlete with a ligament tear--that can be very painful. Also, just like all mechanical moving parts, joints need to be greased so that they are protected when they slide or rub against each other. This is called cartilage. many joints that are regularly moved also have a fluid called synovial fluid that mimics the action of oil in a mechanical joint. 
 
Many joints allow for growth since bones are always growing or being replaced. Some joints allow small movements such as the gliding joints in the back-bone. Each bone can move a little. Actually, one part of the bone glides over another bone and together they can bend your back. 

But there are three basic types of movable joints which allow large movements. 
 
The hinge joint 
 
A simple joint (such as in the knees and elbows) is the hinge which we just saw. Here the movement is only in one direction. Fingers also have hinged joints, so you can bend your fingers only one way. 
 
The figure shows the knee joint. The knee cap is called the patella and restricts movement. At the joint, the lower tibia and fibula bones meet the thigh bone called femur. 
 
The ball and socket joint 
 
The movements in the shoulder or hip are in more than one direction (we can move our shoulders in a circle, but not our knees) and this type of joint is called the ball-and-socket joint. 
 
As you can see from the figure, one bone has a rounded end that fits into a cuplike cavity on another bone. This provides a wider range of movement. Thus, your hips and shoulders can swing in almost any direction. In the hip, the socket is in the hip bone or pelvis and the ball-like head of the femur fits into it (Picture from www.eorthopod.com) 
 
The pivot joint  
 
There was one type of joint none of us guessed: yes, it is the neck joint that allows the head to move side to side (and say yes and no!). 
 
BOX on Bones 
 
The human skeleton has 206 bones. Our bones begin to develop before birth. When the skeleton first forms, it is made of flexible cartilage, but within a few weeks it begins the process of ossification, when the cartilage is replaced by hard deposits of calcium phosphate and stretchy collagen, the two main components of bone. It takes about 20 years for this process to be completed. 
 
The bones of children and young teens are smaller than those of adults and contain "growing zones" called growth plates. These plates consist of columns of multiplying cartilage cells that grow in length, and then change into hard, mineralized bone. These growth plates are easy to spot on an X-ray. Because girls mature at an earlier age than boys, their growth plates change into hard bone at an earlier age. 
 
Bone building continues throughout life, as the body constantly renews and reshapes the bones' living tissue. Bone contains three types of cells:  
 
osteoblasts, which make new bone and help repair damage;  
 
osteocytes, which carry nutrients and waste products to and from blood vessels in the bone; 
 
osteoclasts, which break down bone and help to sculpt and shape it. 
 
Osteoclasts are very active in children and they also play an important role in the repair of fractures.