Today, we try to adapt to a new methaphor. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

In 1994 I suggested the metaphor of mentors and servants when I was asked if computers would replace books. I said they could not, because books and computers had established such different roles in our lives. The book is a metaphorical mentor -- the voice of a teacher. I give myself over to a book when I read it. I may criticize it when I'm done, but the immediate relation between me and my book is that of a student to a teacher.

The computer is quite another matter. From the start it was there to serve us. We go to the internet to be served facts, not to be mentored. We order it to reserve a hotel room or send a message to a friend. When I call up the library's web page, I mix metaphors -- I order it to find which book is there to teach me. But, in the end, we will not allow these roles to stay mixed.

Now, while computers won't replace books, they still harbor mischief. And that mischief shows up in the Engines web site. When I speak on the radio, either you hear me out, or you push the button. What you cannot do is control what you hear -- any more than you can alter the words in a book. If you want to regain control, you turn to your computer. You summon the Engines web page and order it to find a fact or retrieve a script. Behind the microphone I function (metaphorically, at least) as your mentor. On the web, I am your servant.

Now the mischief: several thousand people tap into the site every week, and many send email. Often those visitors have never heard of the radio program. When Yahoo finds an Engines page on, say, Albrecht Haller's poetry or the Colonial use of digitalis, visitors email me. They order up in-depth knowledge of pre-romantic German poetry or Colonial medicine.

Mail pours in from school children, from people who have something to add, from serious academics, and from the idly curious. Each individual email is welcome. In aggregate, they turn me into a kind of information butler -- happy to serve where I can, frustrated when I cannot, and bone tired.

A far more visible person in this predicament is movie critic Roger Ebert. His web site brings in hundreds of letters each day. In obvious desperation, Ebert lists the kinds of letters he won't answer. No help with school assignments, no answers to factual stuff that's easy to find on the web. He won't answer trivia questions, and he won't read your screenplay. He's telling us that he cannot provide the level of service expected of a good servant.

And so, by casting old material in a new medium, the web sharpens the metaphor of mentors and servants. Once there, it awakens new expectations. Technology often does that. Typewriters were no simple replacement for pens, nor airplanes for automobiles. Now and then, a new technology sets us off on an entirely new road. And when it does, we're all left just a little bit breathless.

I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.

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I've used the mentors and servants metaphor not only in Episode 877, but also in keynote addresses to the serials librarians association, NASIG and the Texas Library Association. My thanks to Julie Grob, UH Library, who triggered this program when she pointed me to the Ebert web page.