Dear Dr. Boli: People keep telling me I should get a hybrid to replace my 1973 Chrysler Newport. But I don’t want to get one until I know what a hybrid car is and how they pollinate cars. —Sincerely, Jennifer Granholm, Secretary of Energy.

Dear Madam: The sexual reproduction of motor vehicles is a branch of biology outside Dr. Boli’s field of expertise. Fortunately, however, it is irrelevant to your question, because the word hybrid here is used in a metaphorical sense.

A hybrid vehicle is a failed attempt at a perpetual-motion machine. The turning of the wheels generates power, and the vehicle uses that power to turn the wheels. If perpetual motion were allowed, that would be the end of the story. But in spite of the best attempts of various legislators to repeal the laws of thermodynamics, they are still on the books; and therefore perpetual motion is not an option. Power must occasionally be injected into the system to keep it running, and thus your hybrid vehicle, which desperately wants to drive on electric power alone, also has a gasoline engine. This is the sense in which the word “hybrid” is used: a hybrid is like an electric car crossed with a gasoline-powered car by some ambitious technician at the agricultural extension agency.

It is not, however, necessary to use the gasoline engine all the time, or even at all. There is a source of unlimited energy into which all vehicles can tap, and it does not live under the feet of greedy foreigners who hate us. Gravity is freely available to all. As long as you drive downhill, you can make use of this free energy indefinitely. This one weird trick eliminates the need not only for the gasoline engine, but for the electric motors as well.