To prevent skin cancer, it's important to use sun protection even on cloudy days and to see a dermatologist for annual skin checks.

Wet hair doesn't cause colds

"One patient I saw recently came in complaining of a cough and sore throat. She said she was totally to blame for catching a cold since she'd left the gym with wet hair."

Leah Rothman, an osteopathic physician who practices family medicine in San Francisco, says this isn't the first patient who's presented her with this theory.

There's absolutely no connection between wet hair and getting sick. The myth stems from an Old English saying about "catching your death of cold," she said. When the phrase was coined, people would avoid bad weather by staying indoors and it was the close proximity to other people that would lead to fatal illnesses, such as pneumonia.

"It's likely the playroom where she dropped off her child or the shared gym equipment she used," Rothman said. "Both those places are good sources for germs."

The best defense for avoiding illness is practicing good hand hygiene and wiping down any shared equipment.

Exercise alone isn't enough to lose weight

"The patient tells me he or she has been on the elliptical each day for 45 minutes, and that the machine says 700 calories have been burned. They get on the scale, and find there's been no weight loss."

It's a common myth that exercise alone will help a person lose weight, said Brad Saks, a clinical psychologist who serves as science adviser for Retrofit, a data-based weight-loss program. People tend to overestimate the number of calories they burn and how hard they worked out.

Even if you're on the stair stepper, "machines highly overestimate the amount of calories burned," Saks said. "The maximum calories burned in an hour is 200 to 250, tops."

Saks says that when people exercise more, they tend to eat more.

"They'll make up the deficit quickly," he said.