At this time of year, many people set out to visit relatives in faraway places. You may be looking forward to a trip to your grandparents' house in another city, or to a visit with some old friends who moved to another state. You can't wait for the trip to begin -- except for one little detail. You get carsick.

For the many people who suffer from carsickness, or from other kinds of motion sickness like airsickness or seasickness, travel isn't much fun. Being in an interesting new place is great -- but getting there is terrible. Some people even avoid traveling because of motion sickness.

What causes it?

To understand motion sickness, you have to start with a part of your body hidden deep inside your head, an area called your inner ear. Wait a minute -- inside your head? You'd think that a sickness that made you feel queasy would start in your stomach.

Motion sickness sometimes ends up making you sick to your stomach. But it begins in special parts of your body called the utricle, the saccule and the semicircular canals. These strange-sounding things control your balance. Your utricle and your saccule tell you which way is up. Your semicircular canals keep track of changes of direction as you move around.

All three balance organs are filled with fluid, sort of like the level that a carpenter uses. A carpenter uses a level to see if a piece of wood is flat or tilted. If it's tilted, the liquid in the level will look uneven. Like a level, the organs in your inner ear are very sensitive to changes of the position your body is in. As you move, they send signals to your brain, which in turn sends signals to your muscles. The muscles move into exactly the right position to keep you from falling over.

You have probably discovered what happens when you upset your sense of balance. You can do it by spinning your body around and around. This disturbs the fluid in your inner ear. It keeps moving even after you stop spinning. Your brain keeps getting the message "This body is whirling around and around and around . . ."

So even though your body has stopped spinning, you still feel like you're moving. Or you may feel like you're standing still and the world is moving. This confusing sensation of dizziness goes away quickly.

So what does all this have to do with feeling queasy when you're sitting in the back seat of the car on the way to your grandmother's house in Ohio?

A lot. Even though you're not spinning as you travel along the highway -- unless your parents have a very weird car -- you are in constant motion. Objects outside the car move by, and your eyes dart from side to side, keeping the trees or cows or roadsigns in focus as you go. Your brain has a lot to do: It must constantly process the signals from your eyes, and sort out whether you're sitting still or moving -- when you're actually doing both.

During the trip, your brain gets mixed signals -- and keeps getting them as long as the car keeps moving. Once the signals begin to get confused, you may experience a similar feeling to the one you get when you spin around. The medical name for this feeling is vertigo. But whatever you call it, it makes you feel awful. Some people don't get motion sickness at all. Some unlucky people get it on very short trips -- like a ride on an escalator. People with bad motion sickness may even throw up.

The fast cure for motion sickness is pretty easy to figure out. Just stop moving. If you're on a pitching boat, you'll feel better after a rest stop. If you're airsick, you'll be cured by coming in for a landing.

But you can't avoid taking trips. You wouldn't want to. Doctors have found that some medicines work well to treat the symptoms of motion sickness. Like many drugs, however, drugs that fight motion sickness may not be good for children. Or they may have side effects that are just as unpleasant as the sickness. But for some people who get very bad motion sickness, medicine works very well.

If your motion sickness isn't too bad, try keeping a window open -- if you're in a car -- and listening to the radio to distract yourself. If you feel sick, try lying down and keeping your head still. Don't try to read or look at the passing scenery or watch the horizon. This will just make the poor inner ear and brain more confused. It may help to suck on an ice cube or drink some cola.

In a car, ask the driver to make frequent stops to let your inner ear get back to doing its usual great balancing act. It may not help much when you're feeling carsick, but you can depend on your inner ear to get you back on your feet again fast. Tips for Parents

What's the best preventive measure for car sickness? "Stay home," laughs Dr. Hank Harris, a spokesman for the American Academy of Pediatrics. More seriously, Harris, a Stamford, Ct., pediatrician, suggest that parents treat their kids with Dramamine. "It's safe and effective, and it does work." And while the over-the-counter preparation will make children drowsy, "That's not necessarily a bad thing on a long trip." If you want to avoid using medication, Harris suggests giving your kids cola and soda crackers to settle their stomachs. Keep a window cracked so oxygen will circulate in the car. "And take heart," the physician adds. "Most kids grow out of this problem. They may continue to get dizzy as adults. But that's easier on the car upholstery."