My first bicycle was a used, faded-orange Montgomery Ward Hawthorne with red tires and a coaster brake. Its wheels were half as high as I was and I had to climb onto something to reach the saddle.

It came to a sudden and tragic end one day when a walnut tree leaped out in front of me while I was simultaneously "trick riding" and watching the neighbor girl in whose honor I was making a fool of myself.

It was with some curiosity and unrealistic expectations that 16 years ago, I first mounted a ten-speed, anticipating all sorts of marvels from moving one of its shift levers. I'm a self-proclaimed expert with all those gears and levers now, of course, real shifty, but there are a lot of people, I suspect, still viewing that clanking mechanism with the same lack of understanding.

It's really quite simple. In the front, attached to the pedal cranks, are two "chain-rings," one bigger than the other. In the back there is a cluster of five different-sized "cogwheels." Two chainrings times five cogwheels gives you ten different gearing combinations. You shift into these gearing combinations by moving the chain from one chainring to the other, and from one cogwheel to another.

The shifting lever on the left moves the chain from one chainring to the other. The shifting lever on the right moves the chain from one cogwheel to another. The shifting levers are connected by cables to "derailleurs" which move sideways to "derail" the chain from one chainring or cogwheel to another while you are turning the pedals. You must be tuning the pedals in order to shift.

When the chain is on the largest chainring (in front) and the smallest cogwheel (in back), you are in your highest gear, used for riding downhill or on the straight-and-level. Conversely, when the chain is on the smallest chainring (in front) and the largest cogwheel (in back), you are in your lowest gear, used for climbing hills.

It may help you to remember that, when you shift the chain down onto the smaller chainring in front, you are dropping down into a lower gear.

The cluster of cogwheels in back is somewhat cone-shaped. To climb a hill, the chain must climb up onto the largest cogwheel. When you start downhill again, you drop the chain down onto the smallest cogwheel.

Therefore, if you're rolling down the road -- with the chain on the largest chainring in front and the smallest cogwheel in back -- and you come to an upslope, you move the lever on the right, shifting the chain to one of the larger cogwheels, in back, making it easier to pedal. If the slope gets steeper or you begin to tire, you may wish to move the lever on the left, dropping the chain down onto the smaller chainring, making it still easier to pedal.

As the slope begins to level out and you begin spinning the pedals ineffectively, shift back to the larger chainring and, when it's straight-and-level again, shift bact to the smallest cogwheel in back again.

The front and rear derailleurs must be kept in proper adjustment in order to align the chain with whichever chainring or cogwheel you're shifting to. Fi you try to shift, however, and the chain promptly drops off the chainring or cogwheels, don't sell the bike. It requires only a simple adjustment with a screwdreiver. Your bike shop will be happy to do it for you, or to show you how to do it.

The "serious" bikie refers to the gears in terms of inches, involving an archaic formula I'm not sure I can explain without more space and waking up the math student next door. Let's just say, for easy reference, the lower the gear, the lower number of inches expressed, and the higher the gear, the higher number of inches. That kid next door is pretty surly before noon.

A common high gear would be 100 inches, a common low gear, 38 inches. If you have trouble getting up the hills, you want as low a gear as you can get, perhaps 35 inches or lower. If you're the kind who likes to fly down the mountain, you install the highest gear you can get. In the Tour de France, where downhill speeds reach 60 miles an hour, gears go well above 100 inches.

To determine the lowest gear you have on your bike now, divide the number of teeth on your smallest chainring by the number of teeth on your largest cogwheel, times the size of your wheel (standard wheel size is 27 inches). Example: You have 44 teeth in your smallest chainring, 28 teeth in your largest cogwheel. Forty-four divided by 28 is 1.57. That times 27 is 42 inches. Not low enough for steep hills unless you're Superbiker.

To determine your highest gear, divide the number of teeth on your largest chainring by the number of teeth on your smallest cogwheel, times the wheel size.

The cogwheels on "better" bikes can be changed easily to cluster combinations with more or fewer teeth, permitting you to adjust your gear ratios to whatever kind of riding you contemplate, mountainous or straight-and-level. You can also get larger or smaller chainrings, though these can be quite costly.

The trick is to end up with a combination of gears giving you the high-low range you want without the gaps between the middle gears being too large. It's fun. Count the teeth on your chainrings and cogwheels, get a pocket calculator and a pencil and paper. Lots of paper. And maybe a box lunch.

As you substitute one tooth number for another, trying for the perfect combination, you discover you have two gears with the same number of inches. Now you have a nine-speed bicycle. Scratch that out and start all over.