Since the invention of the wheel, some great mysteries have puzzled travelers.

Where was Atlantis? What do the formations at Stonehenge mean? Why does the Leaning Tower of Pisa lean?

But there are other, evidently unsolvable, travel questions I find even more baffling:

Universal plugs?

Along with ending poverty, war and pollution, why don't the nations of the world get together and agree to use just one kind of telephone and electric plug? There are about a dozen kinds of electric plugs, from the two-flat-pronged U.S. version to such seemingly eccentric varieties as one used in the Canary Islands that has three round prongs in a row.

England has its own three-pinned plug, incompatible with the two-pinned variety used just across the channel in France.

Once upon a time, such inconsistency merely annoyed users of blow dryers and electric razors. But now there are about 40 kinds of telephone plugs to contend with, too, a headache for laptop users. The plugs come in so many bizarre shapes and sizes that you'd need a tool kit to establish dial-up Internet connections on a trip around the world.

Blame the proliferation of plugs on differing standards. "Up until the 1970s, most electric and telephone companies were state monopolies," said Steve Kropla, whose www.kropla.com site helps travelers hook up their laptops on the road. "They just developed their own systems."

AIRPLANE FOOD

Why is it so hard for airlines to serve appetizing meals, even if no more complicated than salads and sandwiches?

These days, you're lucky to get snacks and beverages on an airplane, and even those might not be free. Generally, only passengers on long-haul flights and in first class can expect a hot meal.

Of course, airlines aren't really in the food service business. Their priority is transporting passengers safely. Besides, cooking at 30,000 feet is complicated. Reston, Va.-based Gate Gourmet makes more than half a million meals a day. Ingredients must be carefully selected to avoid spoilage, and cooking techniques must be adapted to tight galleys. Sanitizing tableware and utensils, providing accessories (salt and pepper, condiments, napkins, etc.), and getting the meals on board in time for departure require logistics few passengers consider when griping about the food.

Airlines are installing convection and steam ovens that prevent foods from drying out. Recipes are simpler, emphasizing recognizable ingredients.

"You shouldn't have to ask what it is when you're served something," said Gate Gourmet executive chef Bob Rosar.

Last fall, a survey released by the consumer survey company Zagat gave top ratings for in-flight meals to Aloha, Continental, Hawaiian, United and American. Among the budget carriers, Midwest, Song and JetBlue scored highest for food (but these are often just snacks).

HOTEL ROOMS

Did you ever book a room at a fancy, high-priced hotel, only to find it in an uncongenial setting, with ugly decor, laggard service and claustrophobic rooms or, even worse, bugs, dirt at the hem of the shower curtain, burned-out lightbulbs and minimal soundproofing (treating you to the intimacies of the couple next door)? All that has happened to me, most notably about 10 years ago at a Washington, D.C., hotel once favored by heads of state, where ant traps sat on the window ledge.

HOTEL RATINGS

Who rates hotels anyway? What do the stars mean?

As it turns out, the stars are often self-awarded. "There is no governing body saying what a world-class, five-star hotel is," said Harry Nobles, an industry consultant and former head of the AAA Lodging/Dining Ratings Program. On a recent trip, Nobles found that inconsistency was especially striking in Asia, where some hotels with stars were all but uninhabitable.

As a result, travelers tend to book rooms at hotels that are part of reliable, high-profile chains. "But that's unfortunate," Nobles said, "because there are thousands of wonderful (unrated) independent hotels."

Hotels in Rome bear stars based on a checklist of amenities — elevators, room service, private baths — granted by the self-monitoring Federalberghi Association.

In London, several hotel rating schemes hold sway, including those of the Royal Automobile Club and the National Tourist Board. "But all of them are slightly different, and none are definitive," said Jamie Talmage, a London hotel expert.

The U.S. has AAA, which assesses 32,000 hotels and motels in North America and the Caribbean. Getting an AAA one- to five-diamond rating costs a hotel nothing, except adherence to a basic amenities list and willingness to submit to annual surprise visits by AAA inspectors.

Thanks to the auto club, the U.S. may have the most reliable hotel rating system in the world. "But any system run by humans is going to be imperfect," Nobles said. "Personal opinion is going to enter into it."

STANDING IN LINE

I can think of many more mysteries raised by travel. Why don't people everywhere stand in line and wait their turn? Isn't the first-come, first-served rule written in stone? But in China, I found, queuing up isn't natural; to get a train ticket or steamed bun, it's survival of the pushiest.

In France, people creep around you in line, smoking cigarettes and hoping you'll forget they belong behind. And nobody ever raises a stink when someone sidles to the front, although stoning seems suitable to me.

Why does it take so long to get off a plane after landing?

When will airlines show classic old movies such as "Gone With the Wind" and "The Philadelphia Story" instead of lousy comedies?

What makes tour guides tell dumb jokes?

Such question are, I fear, as unfathomable as the Stonehenge riddle.