People are ridiculously bad at estimating the caloric content of food they eat. We can see the volume of food, feel the weight, taste it but we cannot without practice and training easily guess how many calories it will be.

Also people are just as bad at estimating how many calories they would burn by exercisng or walking.

We've all heard it -- "I'll need to walk around the block after eating that piece of cheese cake" phrase. I've said it myself many times. Even though I know it will take hours and hours of walking around a very large block to burn it off so to speak.

One more thing. There is a negative aspect of exercising in relation to weight that is often overlooked. Because of the pervious 2 points, people who do exercise will rationalize eating more food because "they will burn them off in 15 min on treadmill".

Now I am using "calories" here in a simplistic way, but we had long discussions before about how it is not as simple as calories and calories out. It is bit more complicated -- hunger, hormones, insulin level, fat storage rates etc.