Q. How come we don't have flying cars?

—Ari H.

A. According to Paul Ceruzzi of the Smithsonian Institutions Air and Space Museum in Washington, D. C, the main reasons we don't have flying cars is that airplanes are harder to "drive" than cars, they cost more, and they burn more fuel. All that could change, though. On-board computers and satellite navigation systems may make flying as easy as driving is now. New materials, better designs, and more efficient engines may make planes cheaper to buy and to run, as well as quieter. "The gap is closing," Ceruzzi says.

In Davis, California, an engineer named Paul Moller is testing the Skycar, a flashy flying automobile with eight rotary engines, two computers, and (just in case) two parachutes. If it passes its tests, he hopes to build Skycars to sell. Don't start begging your parents for one just yet, though. The first models will cost a million dollars each, you'll need a pilot license to fly one, and you'll have to take off and land at airports.

—Robert

Q. Could you please tell me how to collect samples of bacteria from toilets, tables, etc., and put them in petri dishes?

—Becky B.

A. I asked several microbiologists, and they all agreed that growing the stuff from your toilet is not a good experiment to try at home. Some bathroom bacteria are dangerous, and you really need a lab to keep them under control and dispose of them safely. You're better off starting with the bacteria and fungi on your own skin. The Explorabook (Klutz Press) tells you how to grow and study the things that live between your toes. You can also get microbiology kits from science-toy stores in many shopping malls. If you try this, be careful. Make sure you work with an adult and wash your hands a lot (you'll want to, once you see what's on them).

—Robert

Q. Can you give me a list of upcoming solar and lunar eclipses?

—Celia H.

A. First, a quick review. A solar eclipse happens when the Moon casts a shadow on part of the Earth's surface. If you're standing in the shadow, the Sun will seem to disappear either partly (a partial eclipse) or entirely (a total eclipse) before it comes back again. A lunar eclipse happens when the Earth casts a shadow on the Moon and blots it out. Solar eclipses take place during the day, lunar eclipses at night. The next total solar eclipse visible from the U. S. isn't until 21 August 2017. However, you can see a total lunar eclipse on 21 January 2000. For a complete list of eclipses, go to NASA/Goddard Space Flight Center's Eclipse Home Page (http://sunearth.gsfc.nasa.gov/eclipse/eclipse.html).

—Robert

Q. You printed a photo of a cow named Lady and her clone, Elsie, but they had different markings on their bodies. If they're clones, shouldn't they look exactly alike?

—Cory R., Dave B., Bamey W., and Taj M.

A. Sorry—you haven't unmasked the scientific fraud of the century. As David Wells (one of the scientists who cloned Lady) explained when I asked him your question, all Elsie's coat shows is that genes don't control everything. A calf gets its coat pattern from hair-growing cells in its skin. Each cell produces hairs of a single color. The cells divide and spread across the calf's body while it is still developing inside its mother. Where they end up depends partly on the animal's genes, partly on chemicals in the mothers body, and partly on chance. So even though Elsie and Lady have the same genes, their coat-coloring cells wound up in different places, causing the animals to look different. Elsie may be a clone, but she is very much her own cow.

—Robert

Q. Why is outer space so dark?

—Natalie L. and Katherine H.

A. "Dark" is putting it mildly. Here's how Eugene Cernan, the last astronaut to walk on the moon, described outer space: "When the sunlight comes through the blackness, it's black. I didn't say it's dark, I said black. So black you can't even conceive how black it is in your mind."

It's black because, as Cernan said, sunlight comes through it. Here on earth, the sky can look bright even if you are facing away from the sun. That's because the sunlight bounces off molecules in the air, so that your eyes see light that's not aimed right at them. Outside the atmosphere, there aren't many molecules for it to bounce off. So instead of bouncing back to you, light shoots straight past. As a result, very little of it reaches your eyes, so space looks black.

Q. Was there a year zero, or did the years go from 1 B.C. to A.D. 1 without passing through zero?

—Stella F.

A. On the calendar we use, the year before A.D. 1 is 1 B.C., or –1, with no zero in between them. So if you went back in time 1999 years before the year 1999, you'd wind up in 1 BC. Sound tricky? Astronomers agree. When they calculate what the sky looked like long ago, they count years the scientific way, with a zero between 1 and –1. It's unlikely that the rest of us will switch to their system, though, because we would have to change the dates in too many history books.

—Robert

copyright © 1999 Robert Coontz