Here’s my Q&A column from the WSJ this week — and if you have any questions for me, you can tweet them to @danariely with the hashtag #askariely, post a comment on my Ask Ariely Facebook page, or email them to AskAriely@wsj.com.

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Dear Dan,

I just got back from a trip to Europe, and although I knew that Europeans were much less obese than Americans, it was still shocking to see the difference. It is also not true that they don’t have fast food joints. Can you shed some light on these national differences? 

—Alvin

Some think that the key factor is the European diet: more homemade food and less prepackaged food, smaller portion sizes, less sugar and corn syrup, etc. I have no doubt that there is some truth to this, but I would propose that our differences in weight also have to do with the fact that Europeans use kilograms while Americans use pounds.

Here is my proposed logic, using me as an example: I weigh 170 pounds, which is also 77 kilograms (well, the truth is that right now I might be closer to 174 pounds, but my real weight is 170). Depending on the time of day and what we eat, our weight fluctuates by a pound or more, as most of us know. This kind of fluctuation lets us convince ourselves that when the scale shows 172, our real weight is still 170, even if it has not shown 170 for a while.

If one day our weight is 174, would we say to ourselves “I am gaining weight, and I need to change what I eat” or would we be able to justify this as part of the random fluctuation around our supposed real weight of 170? By contrast, if we were using the kilogram system, the fluctuations would be much smaller, and when we learned that we were one kilogram heavier, we might act on this change more quickly.

My suggestion: Switch to kilograms (and while we’re at it, maybe we can move to the metric system more generally).

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Dear Dan,

There are people in my office who have a hard time focusing for even 20 minutes on their jobs. Nevertheless, they seem perfectly capable of exercising for long stretches, and they are quite persistent in that. Can you explain this contradiction?

—Michael

This might actually not be a contradiction but rather, as I learned recently, two faces of the same mechanism. A few weeks ago, I flew to California for some meetings. I left home at 4:30 a.m. and got to San Francisco at 10 a.m. I had a few meetings, and by 5 p.m. was exhausted. I had a lot of work-related tasks and was determined to get at least some of them done, but I felt devoid of energy. So I went for a run.

Ordinarily, I go for a run maybe once every 10 years. But this run was fantastic! I ran a bit, walked a bit, listened to music along the way. It was challenging, and I ran out of breath, but in no way was it even close to the mental exhaustion of doing the things I was supposed to work on.

Here is my new understanding: I think that people who either don’t enjoy what they’re doing for work or don’t have the mental stamina to focus on it can take long breaks for exercise. On top of that if your co-workers took a two-hour book or movie break, they would be seen as selfish slackers, wasting time. But because society tells us that exercising is good for our health, it is a perfectly good excuse to escape work. Now that I have discovered this way to take time for myself and not feel guilty about it, I am going to do more of it.

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Dear Dan,

The health-care benefits provided by my employer have just been updated to include a hefty monthly surcharge to smokers. I put little effort into quitting smoking, although I know this is the right thing to do. This smoking tax might motivate me to quit, but at the same time it infuriates me that my employer has the power to charge me for smoking. What is your opinion?

—Anonymous 

The smoking rate in the U.S. is about 20%, and companies that add such smoking surcharges usually find that the smoking rate drops overnight to less than 10%. Or, more precisely, they find that the smoking surcharge dramatically reduces the number of people who say that they smoke.

See the original article in the Wall Street Journal here.