Most of us don't have any idea how many calories we're actually eating. Don't believe me? Track your calories for a week or so on something like My Fitness Pal or SparkPeople, and be honest about serving sizes (actually measure out an ounce of chips or a serving of chicken). Unless you're already quite health conscious, you'll be surprised by which of your favorite foods are calorie bombs and how many of those you're eating in a week. I almost cried when I found out my favorite coffee drink packs as many calories as I should be eating in a good-sized meal (but it did explain to me why my jeans were getting so tough to button).
It helps to know this, though, because if you know what you're taking in and putting out, you can make better choices because you're conscious of what you're eating. For example, one of those big bakery muffins? If you weigh about 150 pounds, you'll have to walk briskly for 90 minutes to burn it off. Choose two scrambled eggs and a strip of center cut bacon, though, and you're looking at a 30-minute walk to balance it out. A fast food cheeseburger would take 65 minutes of stationary biking to burn off; a salad with two tablespoons of vinaigrette would only take you about 20.
I'm a big fan of calorie tracking...even if you're not trying to lose weight it helps you keep track of how you're balancing calories and if you're getting the right mix of nutrients. Smartphone apps like the two above are great for this since you'll always have your phone handy to record what you've eaten before you forget. No smartphone? There are plenty of websites that do the same thing. Poke around to find one you like and will use; you'll be surprised how much the information changes your choices.
Do you track your calories? Which app or site do you use?