Body Tricks

By CBSNews.com staff CBSNews.com staff

June 1, 1999 / 10:23 AM EDT / CBS

Sometimes our bodies are capable of some pretty strange things, like making cracking sounds.

In the first segment of the CBS This Morning series Body Tricks, Health Contributor Dr. Dave Hnida of CBS Station KCNC-TV in Denver, Colo., unravels some of the mysteries of the human body.

Cracking knuckles. The reason knuckles make that cracking or popping sound: When the knuckle joint is moved or stretched, the pressure in the liquid in the space between the bones drops, and a little gas bubble is created. When that bubble pops into existence, it sounds like a crack or a pop.
There is no hard evidence to show that cracking your knuckles causes arthritis. But some studies have shown that it causes swollen joints and can weaken the grip over time. That possibility, combined with the fact it annoys a lot of people, might make it a good idea to keep it to a minimum.
Double-jointed. Those who are double-jointed are capable of excessive movement in certain joints because their ligaments, skin and joints are looser than normal.
Photo red-eye. Have you ever snapped photos where everyone in the pictures turn out with red eyes that make them look possessed?
What you're actually seeing is the red blood vessels in the back of the eye (or the retina) that are illuminated by the flash of the camera when the picture is taken.
It's the same principle eye doctors are using to look at your eye when they shine the light into them during a checkup. And that's why newer cameras angle the flash toward the side or flicker a little to avoid the direct light into the eye.
Deja vu. Did you ever wonder about that feeling you've been somewhere before, known as deja vu? Researchers believe that deja vu occurs when you're fatigued or under a lot of stress, and the two sides of your brain temporarily go out of synch.
So, when the right side sees something, the left side doesn't see it or interpret it until a fraction of a second later. That's why you feel like you've been there before. Really, you have.

Body tricks are interesting. But some unusual symptoms might be a sign of something more serious. If you start to experience persistent symptoms, you should see your doctor to make sure you're okay.