Billions of people deal with a nail-biting habit at some point in their lives. Many will go to great lengths to try to stop. And while not all of us are nail-biters, most of us do have a habit we'd like to kick. So what's the best way to break one?

Research shows that intentions alone often fail to lead to long-term behavior change. This isn't to say you can't break a habit. Rather, by understanding the basis of habits, you can create better plans for changing them. For example, we know habits are often affected by environments and routines. Lying in bed may cause you to endlessly scroll through your phone, or watching TV on the couch may lead you to grab a sugary snack.

One of the most effective ways to manage behavior is to identify these locations or times of day. Then try to modify them by changing your routine or creating obstacles that make it more difficult to perform the habit in that space. Moving, switching jobs, or even starting a new schedule, are particularly great times to break a habit or build a new one.

One 2005 study tracked university students' exercising, reading, and TV-watching habits before and after they transferred schools. When students were no longer around old environments and routines, their habits, even the strong ones, significantly changed.

For behaviors like nail-biting, a practice called habit reversal training can be helpful. Developed by psychologists in the 1970s, the aim is to change a habit by replacing it with another one that's less detrimental. The training requires you to analyze and understand your habit cues, so you can effectively step in at the right times. For example, if you tend to bite your nails at work, keep a fidget toy at your desk in advance. Then, if a stressful email comes in, use the toy when you feel the urge to bite your nails.

Breaking a habit takes time, so remember to give yourself grace and have patience through the process.

We often think about relationships on a scale from positive to negative. We are drawn to loving family members, caring classmates and supportive mentors. We do our best to avoid the cruel uncle, the playground bully and the jerk boss.

But the most toxic relationships aren't the purely negative ones. They're the ones that are a mix of positive and negative. We often call them frenemies, supposed friends who sometimes help you and sometimes hurt you. But ifs not just friends. It's the in-laws who volunteer to watch your kids but belittle your parenting. The manager who praises your work but denies you a promotion.

Everyone knows how relationships like that can tie your stomach into a knot. But groundbreaking research led by the psychologists Bert Uchino and Julianne Holt-Lunstad shows that ambivalent (矛盾情绪的) relationships can be damaging to your health — even more than purely negative relationships.

I had assumed that with a neighbor or a colleague, having some positive interactions was better than all negative interactions. But being cheered on by the same person who cuts you down doesn't reduce the bad feelings; it increases them. And it's not just in your head: It leaves a trace in your heart and your blood.

Even a single ambivalent interaction can cause harm. In one experiment, people gave impromptu speeches on controversial topics in front of a friend who offered feedback. The researchers had randomly assigned the friend to give ambivalent or negative comments. Receiving mixed feedback caused higher blood pressure than pure criticism. "I would have gone about the topic differently, but you're doing fine" proved to be more distressing than "I totally disagree with everything you've said."

The evidence that ambivalent relationships can be bad for us is strong, but the reasons can be harder to read — just like the relationships themselves.

The most intuitive reason is that ambivalent relationships are unpredictable. With a clear enemy, you put up a shield when you cross paths. With a frenemy, you never know whether Dr. Jekyll or Mr. Hyde is going to show up. Feeling unsure can disrupt the body's calming system and activate a fight-or-flight response. It's unsettling to hope for a hug while also preparing for a likely quarrel.

Another factor is that unpleasant interactions are more painful in an ambivalent relationship. It's more upsetting to be let down by people you like sometimes than by people you dislike all the time. When someone stabs you in the back, it stings more if he's been friendly to your face.

My plan to learn to play the cello (大提琴) was a health y choice. So I took the next innocent step of going to a shop that sells and rents string instruments, especially violins, violas, and cellos. The place was a hole-in-the-wall, up a steep flight of stairs. Instruments left a narrow path to the counter where a woman with grey hair and warm eyes greeted me. Since I couldn't form a question about what I wanted, I just stood there exploring the scene, which felt immensely pleasurable.

I rented a cello, a bow, and a case to hold them. What attracted me to the cello was its enormous size and its soft, smooth, and memorable sounds. It's an instrument made of beautifully polished wood that I could wrap my arms around and feel its powerful vibrations when the strings were played. That was a good starting point for me. The only problem was that I didn't know how to play it.

There's a popular belief that the cello is a particularly difficult instrument to learn. Another is theadmonition, "Don't bother if you are a beginner over age 10!" Well, I was 70, and what others thought no longer influenced me. And besides, I thought of the words of Dr. Cohen, who suggested that learning causes physical changes in the brain.

So I found a teacher who had respect for older adult beginners and I practiced diligently, daily for years, and sometimes disheartened, but I kept up. I'm happy to report that now, more than a decade later, I can hold my own in a string trio and two quartets （two violins, a viola, and me, the female cello）and even a senior community band. Of course, I'll never sound like Yo-Yo Ma but you could recognize a Mozart piece if you heard me play it. And, more importantly, I don't need to please anyone but myself.

Whether it's a pleasurable pastime or a new career, the starting point is the same: wonder, curiosity, determination, and the desire to keep your brain sharp.