Jim Klestinske, age 12, of Gray Summit, Mo., for his question:

HOW DOES A BICYCLE STAY UP ON TWO WHEELS?

Bicycle riding and racing are favorite activities in just about every country in the world. Cycling has been an Olympic Games event since 1896. The first bicycle was actually a wooden scooter that was made in France in 1790. About 20 years later it was improved in Germany with the addition of a steering bar connected to the front wheel. Another 20 years later from Scotland came pedals. The first U.S. patent was issued in 1866.

A bicycle's two wheels are mounted on a metal frame, one behind the other. Today's bikes have pedals connected to sprockets that are called chain wheels. A chain fits the toothed wheel and extends to a smaller sprocket on the rear wheel. Then the pedals turn the large sprocket which moves the chain, and it in turn moves the small sprocket and the back wheel.

And away we go, steering with handles that are attached to the front wheel.

What keeps you up? Your balance, first of all. It takes practice, and often a learner's bicycle will have two extra wheels attached to the large back wheel to help a person keep his balance.

But the real secret of what keeps you up is the gyroscopic force. When you're riding a bicycle, it's almost as if you were riding a gyroscope. You're able to keep the bicycle standing in an upright position so long as you keep the wheels spinning. You also have the gyroscoping forces of inertia and precession working for you.

Gyroscopic inertia is the ability of a spinning axle to always point in the same direction no matter how the support of the gyroscope moves about. Precession is the tendency of a gyroscope to move at right angles to the direction of any force applied against it. Precession makes the bicycle turn a corner when you lean to one side.

You stop most lightweight bikes by using caliper brakes which are operated by squeezing levers on the handle bars. The levers operate sets of rubber brake pads called shoes, located on both front and rear wheels. The shoes press against the rims to slow and then stop the bike.

Standard or middleweight bicycles operate with coaster brakes on the rear wheel. You stop this type of bike by pushing backward on the pedals.

Safety regulations are perhaps the most important part of operating a bicycle. When riding in the street, stay on the right side. Never race, fool around or clown on the street. Learn hand signals and obey traffic signs. Don't ever be tempted to hitch a ride on the back of a truck. While perhaps you have more rules to observe in the city, it is also important for country riders to learn and observe the safety code.