Kevin McKay, age 10, of Bellingham, Washington, for his question:

Why do light bulbs burn out?

This may be hard to believe, but the problem is caused by zillions of traffic collisions in a fantastically busy bottle neck. It is hard to believe because the whole thing is on a miniature scale that is much, much too small for human eyes to behold.. The colliding speeders are electrons.

Electricity, as we know, is created by minuscule electrons. Its energy is created by zillions of them marching along in step, faster than fast. Normally, shells or rings of high speed electrons swarm around the central nucleus of the average atom. A generator or a battery can shake numbers of them loose and send them racing in step through an electrical wire circuit.

The electrical wires in your home are part of an unbroken wire circuit that runs to and from a generator. Here and there we install outlets where we can draw helpings of the electrical energy carried along in the big power circuit. Household appliances are designed to use the energy of speeding electrons to work toasters, steam irons and such. The most useful outlets turn night into day with electric lights. We can switch them on and off many tames, but no electric bulb lasts forever.

Sooner or later it burns out and leaves us in the dark. This is because those speeding electrons finally destroy the delicate wire filament inside the bulb. The metal base is screwed into a socket outlet. When we switch on a reading lamp, it connects to the electrical energy in the giant circuit. Streaming electrons zoom though the wires in the bulb.

Two large prongs connect to the metal base and their tops are connected with a delicate coil of fine wire. There is room for the speeding electrons to travel through the prongs in comfort. But suddenly the traffic is forced helter skelter through the thin filament at the top. This creates a bottle neck and a fantastic traffic jam. Billions of electrons bash together and bounce apart. These collisions create enough heat to make the filament white hot    and glow with its bright electric light.

But when metals get red hot, their atoms and molecules tend to separate and break apart. Some metals resist heat better than others. The filament in most bulbs is heat resistant tungsten. But sooner or later the heat from colliding electrons breaks its atoms apart. Then the weary wire breaks and the electric circuit is broken. The light bulb is burned out and useless.

It is hard to imagine the speeds and numbers of miniature electrons in this fantastic traffic jam. To light an average reading lamp, every second about three billion billion electrons jog back and forth sixty times. In the crowded bottleneck, billions of them bash and bounce apart, creating the heat that makes the filament glow. Oxygen would make the filament burn faster. So the wires are sealed in an airtight bulb to help the delicate wire to last as long as possible.