Ever wonder how to stop your eyes from tearing when you chop an onion, or how to get rid of bad breath? Well, look no further. We asked top food scientists to help us solve these and other mysteries.

Q. Why is it that when you and your partner both eat garlic, you don't smell it on the other person? Also, why is it so difficult to get rid of garlic breath?

A. According to Luke LaBorde, a professor of food science at Penn State University, "You probably don't notice the smell because your olfactory system is saturated and your brain no longer receives 'garlic signals.' It's the same as if you worked in a horse barn: After a while you don't notice the smell."

It makes sense. Clearly, we can be desensitized to our own bad breath.

"So, it would make sense that if we had bad garlic breath, and we became desensitized to it, we would similarly become desensitized to our partner's simultaneous garlic breath," says Stephen J. Pintauro, a professor at the University of Vermont.

One of the reasons that garlic breath is so strong is that it's "full of sulfurous compounds that 'feed' the bacteria in the mouth, and bad breath (halitosis) is caused as a result," says Dr. Gerard E. Mullin, director of gastroenterology at Johns Hopkins Hospital.

The more relentless bad breath associated with garlic originates in the intestinal tract: "Sulfur-containing gases from the garlic are produced in the intestines and are only slowly eliminated by metabolism," says Pintauro.

LaBorde also notes that the chemical responsible for garlic breath enters the bloodstream and circulates. "The volatile garlic compounds diffuse from the blood to the air deep within the lungs, and we breathe them out."

Q. Why does parsley counteract bad breath?

A. Chlorophyll — the green pigment in plants responsible for photosynthesis — gets rid of bad breath. Chlorophyll tablets have been recommended for reducing odors, but their effectiveness has not been entirely proved. The exact mechanism is not known, says LaBorde.

Q. Why do you tear up from an onion? And how can it be stopped?

A. According to Pintauro, when you slice an onion, you disrupt cells in the vegetable that contain an enzyme (lachrymatory-factor synthase) that catalyzes the production of volatile irritants. The synthase enzyme converts the sulfoxides (amino acids) of the onion into sulfenic acid. The unstable sulfenic acid then rearranges itself into syn-propanethial-S-oxide, which gets into the air and comes in contact with your eyes.

"Once it hits the eye, it turns into a mild form of sulfuric acid, which causes burning," adds Don Schaffner, a food science professor at Rutgers University. The lachrymal glands become irritated, and the eyes tear.

Pintauro recommends chilling the onion before chopping. "Enzyme activity is greatly reduced at lower temperatures. So, you can minimize the production of the volatile irritants by chopping cold onions. Also, I have heard that simply wearing swimming goggles may help."

Swanson has a few other suggestions, including: "Wear contacts; peel and slice under running cold water or close to running cold water to aspirate the volatiles away with the water; light a candle before chopping and chop onions in proximity to the candle so the tear-causing volatiles will be volatilized in the rising heat from the candle and away from your eyes."

Q. What in asparagus causes a person's urine to smell?

A. "The smell is likely due to the normal metabolism of certain amino acids that are high in asparagus," says Pintauro.

There is some debate as to exactly which sulfur compounds and which amino acids are involved, but it clearly is a sulfur-related smell.

"One of the compounds identified is methyl mercaptan, he adds. "This chemical smells like rotten cabbage. One of its industrial uses is as an additive to natural gas, so that you will smell it if there is a gas leak or if you've accidentally forgotten to turn off a gas burner."

Q. Why does spinach make your teeth feel funny?

A. "Spinach, like many other plants but more so, contains very small crystals of oxalic acid as a waste product of the plant's metabolism. They are not very soluble in water, and one can sense them as a very fine grit on the teeth and tongue," says LaBorde. And although oxalic acid can be harmful, the amount in spinach is too small to do any damage.

Q. Does salt make water boil quicker?

A. No.

"Actually, the effect of any dissolved solute, including table salt, is to increase the boiling point of water,"says LaBorde. "So, water containing salt should take longer to boil than water with no added salt."

And according to Barry Swanson, professor of food science at Washington State University, this phenomenon is the same reason salt is spread on icy roads during winter — to lower the ice's melting point and keep the ice off the roads.

However, there is one reason that salt might appear to increase the speed with which water boils, he said: "Salt decreases the surface tension of water so that smaller, more foamy — and thus noticeable — bubbles can be formed. Without salt, fewer and larger bubbles are formed. Perhaps this observation is confused with faster boiling time."

Charles Stuart Platkin is a nutrition and public-health advocate, and author of "Breaking the FAT Pattern" (Plume, 2006). Sign up for the free Diet Detective newsletter at www.dietdetective.com.