Michelle Trudeau

Bears do it; bats do it. So do guinea pigs, dogs and humans. They all yawn. It's a common animal behavior, but one that is something of a mystery.

There's still no consensus on the purpose of a yawn, says Robert Provine, professor of psychology and neuroscience at the University of Maryland, Baltimore County. Provine has studied what he calls "yawn science" since the early 1980s, and he's published dozens of research articles on it. He says the simple yawn is not so simple.

"The lion and calf shall lie down together," Woody Allen once wrote, "but the calf won't get much sleep."

Katharine Hepburn had it. So did playwright Eugene O'Neill and Sen. Robert Byrd. Essential tremor is a condition that causes involuntary shaking.

While it usually develops in middle age, it can start much earlier. Shari Finsilver was aware of her hands shaking as a child.

Most research on memory loss in the elderly focuses on dementia, Alzheimer's disease or other brain diseases.

But neuroscientist Emily Rogalski from Northwestern University's Feinberg School of Medicine knew there is great variation in how good memory is in older people. Most have memory loss to varying degrees, but some have strong memories, even well into old age.

Miranda Kelly, a 14-year-old from Sykesville, Md., says she's been sleepwalking since she was 6 or 7. The first time, she says, "I woke up on the couch on a school day. And I'd gone to bed in my bed."

Since that first episode, Kelly now sleepwalks every couple of months. "I wake up in weird places, randomly. I have once woken up in the kitchen, and on the floor of the bathroom wrapped in my sheet," she says.

Six years ago, we told you about a woman, identified as A.J., who could remember the details of nearly every day of her life. At the time, researchers thought she was unique. But since then, a handful of such individuals have been identified. And now, researchers are trying to understand how their extraordinary memories work.