Dear Doctor: The 7-year-old in our family has discovered yawns are "catching," and he just loves it when he can get his daddy to start yawning, too. Of course, now he wants to know why, but even after looking online, we're not sure of the answer. Does anyone really understand yawning?

Dear Reader: True confession -- reading your question triggered a yawn. (OK, two.) That's not unusual, as it's estimated that for well over half of us, yawns are contagious. In a study at Duke University, a video of people yawning had close to 70 percent of the 328 participants doing the same during the three-minute showing, some as many as 15 times. Lest we think this is a humans-only phenomenon, contagious yawning is also a hallmark of chimpanzees and a group of primates known as Old World monkeys. And as dog owners can attest (yes, there are studies into this as well), it's a trait also shared by many of our canine companions.

So why do we yawn and why are they catching? Though these questions have tantalized scientists, philosophers and all of us yawners for millennia, we're still short on definitive answers. As far back as 400 B.C., Hippocrates pondered the origins of the spontaneous yawn. That is, a yawn that occurs without the prompt of someone else's yawn. He associated it with a general reflex to cool the body, which turns out to be a decent guess. Other theories put forth to explain the spontaneous yawn have included drowsiness, boredom, weariness and empathy. More recently, researchers have come to see potential for the yawn to be used as a diagnostic marker of neurological disease. To that end, there's now a specially designed yawning susceptibility scale to measure exactly how prone someone is to "catching" a yawn.

In recent years, researchers have identified a link between temperature and yawning, thus giving Hippocrates' theory from 2,000-plus years ago a nice boost. In one study, 120 pedestrians selected at random during both hot and cold weather were found to "catch" a yawn more frequently within a certain window of warmer temperatures. In another experiment, researchers were able to affect the rate of both spontaneous and contagious yawning with the use of cold and hot packs. Variables like the person's sex or age, how much they had slept the night before, time they spent outdoors, humidity and the season of the year didn't influence their yawning behavior.

Last year, researchers in England found a connection between spontaneous yawning and the primary motor cortex, a region of the brain that takes a lead role in generating the messages that initiate our physical movement. They also discovered that trying not to yawn actually increases the sense of needing to yawn. According to the researchers, these findings may help shine a light on conditions associated with impulse control, such as obsessive-compulsive disorder or Tourette's syndrome.

Thus far, research into yawning has added tantalizing bits and pieces of information about a simple action with complex origins. That's why, despite your best efforts at research, a definitive answer eluded you. Chances are, though, you yawned while reading this column. And if he was in the room with you, so did your 7-year-old.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctor: I'm only 22 and already I've gotten whiplash from all the studies about coffee. First it's bad for you, then it's good. Then, oops, no -- it's actually bad. My girlfriend's a serious coffee drinker, and she's thrilled about another new study that says coffee's going to help her live longer. Is that right? How do we know what to believe?

Dear Reader: We agree that the back-and-forth about coffee over the decades has been confusing and for coffee drinkers who want a final answer, frustrating. Part of this is because of the nature of ongoing research, which, as it asks new questions, incorporates the newest data. Add enough variables to any line of inquiry, and chances are quite good that the conclusions will shift, if not change.

Another factor that plays a role is the study itself. Many of the coffee studies, including this latest one, have been observational studies. That means that researchers gather data from large populations, identify and account for lifestyle or environmental factors that could sway results, then analyze the resulting data to draw conclusions. When they're finished they have a correlation, but not a definitive cause. That doesn't mean such studies should be discounted. Far from it. It was through observational studies that researchers first linked smoking to lung cancer. This in turn led to the more rigorous and targeted research that revolutionized how we view tobacco and tobacco products.

We suspect that coffee is the subject of so many studies for a couple of reasons. First, it's so widely consumed. Here in the United States, it's our favorite beverage. We drink more coffee than soda, tea and juice combined. Plus, thanks to its caffeine content, coffee is a stimulant. In fact, caffeine is the most widely consumed physiological stimulant in the world. That's why, with regular use, it can result in a mild form of physical dependence. In addition, caffeine has been associated with adverse side effects in some individuals, such as temporary spikes in blood pressure. All of this -- widespread use, potential physiological effects, as well as the numerous bioactive compounds that it contains -- have made coffee a prime target for research. Which brings us to the new study now making headlines.

Researchers in Britain looked at a decade's worth of health data for about 500,000 adults who regularly drank from one to eight cups of coffee per day. This included brewed and instant coffee, as well as decaf. According to their analysis, those who drank coffee regularly had a slightly (emphasis is ours) lower risk of death than did non-coffee drinkers. Although the study didn't address questions of how or why, the researchers have cited coffee's complexity.

In addition to the caffeine that reels us in, coffee contains over 1,000 different chemical compounds, including B vitamins, potassium, magnesium, as well as hundreds of phytochemicals with antioxidant and anti-inflammatory properties. However -- and this is important -- if you're not already a regular or heavy coffee drinker, don't dive into the deep end with a multi-cup habit. And when it comes to pregnant women, the consensus is clear -- severely limit (or quit) coffee. That's because the enzyme needed to metabolize caffeine is not present in the fetus.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctor: I’ve never really enjoyed exercise and have managed to get to a fairly healthy 58 without doing very much of it at all. But I just got remarried and my husband, who is pretty active, wants me to start some kind of fitness program. Really, at my age, what good will it do?

Dear Reader: We’re sorry to rain on your no-exercise parade, but your husband is on the right track. (Congratulations on your marriage!) It’s never too late to start to reap the benefits of a fitness program. It’s not that we don’t understand your wish to avoid it. Even the most ardent workout enthusiast has been faced with the twin challenges of physical discomfort and boredom. But we think half the battle when easing into a fitness program is choosing the right activities, then setting modest goals. Exercise comes in many guises and with some creative thinking and a bit of grit, we believe you can find an activity or two that you’ll enjoy enough that you’ll stick with it. But first let’s talk about why it’s a good idea.

We’ve all heard a lot about the psychological boost that exercise imparts, which is no small thing in this stressful day and age. Staying active also helps regulate body weight and contributes to overall good health. Now, new research shows that for people over the age of 50, a time of life when blood vessels start to stiffen up and hearts gradually begin to get less efficient, regular exercise can reverse these effects.

In a study published in January, researchers in Texas evaluated the hearts of middle-age adults, looking at how stiff the cardiac muscles had become. When the study participants were sorted by their degree of physical activity, it emerged that the heart muscles of regular exercisers were stronger and more supple -- and therefore effectively younger -- than those of either the sedentary individuals or those who exercised only occasionally.

Next, the researchers wanted to know what effect starting an exercise program later in life might have on the heart. To that end, they tracked two groups of previously sedentary individuals. One group began exercising for 30 minutes at least four times per week. The other started a program of balance and stretching. Two years later, tests showed that the hearts of the exercise group had become not just stronger but also more supple. The stretch-and-balance group did not reap the same cardiac benefits.

The idea that we can return our hearts to a more youthful state, even later in life, is an exciting one. The challenge is to find an activity (or activities) you’re willing to do at least four times per week and will stick with over the years. Brisk walks or hikes require only a pair of decent shoes. Mix it up with activities you may have enjoyed as a child, such as cycling, swimming, trampolining or skating. Working out with friends, your husband, or with music and books on tape helps make it all more interesting. Just be sure to start slow and take the time you need to ease into your new routine.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)