– Daniel Johnson, San Diego

ANSWER: Freezing preserves food by interfering with microbial activity. But decay due to microbes is not the main problem for most medicines and nutritional supplements. Instead, over time, chemical reactions cause degradation of the drug substance, the nondrug ingredients or even the container, which may leach chemicals into the medicine.

Reaction with oxygen in air (oxidation) and reaction with water (hydrolysis) are common modes of breakdown. Exposure to light, heat and high humidity can increase the rate of drug decomposition. Accordingly, the bathroom medicine cabinet is a bad place to store medicine.

At least 90 percent of a medicine's original potency must remain prior to its expiration date. Estimates of a medicine's shelf life are based on standard conditions, but breakdown may be faster or slower, depending on the actual storage conditions. Breakdown products can be toxic, and their identity can vary depending on how the medicine is stored.

Proper disposal of unused medicines is critical because drugs are now widespread in waterways. Even if their concentrations are too low to affect humans, they may affect fish, and residues of antibiotics might encourage drug resistance in bacteria.

The Environmental Protection Agency recommends disposing of medicines in the trash after mixing them with an undesirable substance such as kitty litter.

My father, who is a retired naval navigator, told me he remembers seeing a sunset and the beginning of a sunrise at the same time. He said he was on a ship somewhere in the Pacific off Hawaii. I think this is impossible unless you were close to one of the poles. Is it possible?

– Jerry Manckia, El Cajon

For a person to see the sun setting and coming up at essentially the same time, the sun must be above the horizon when it reaches its lowest point in the sky and starts to rise. The only places this occurs are within the Antarctic Circle or Arctic Circle around the time of their summer solstices.

Your dad may not have been pulling your leg, though. Perhaps what he saw was a mirage of a double sun. Although rare, double-sun mirages have been reported when the sun is near the horizon and sunlight is refracted through a “lens” of low-density air. The secondary image usually appears above the sun.

Sherry Seethaler is a UCSD science writer and educator. Send scientific questions to her at Quest, The San Diego Union-Tribune, P.O. Box 120191, San Diego, CA 92112-0191. Or e-mail sseethaler@ucsd.edu. Please include your name, city of residence and phone number.