I learned some interesting facts about body fat at this site:
http://www.howstuffworks.com/fat-cell.htm
For instance, a fetus develops its fat cells during the third trimester of pregnancy and then the rest of our fat cells grow at puberty. From then on, we don't get any additional fat cells - they just get bigger!
Our bodies find it much easier to store fat we consume as body fat than it does carbohydrates, so the first choice for fat storage is the fat we eat (this seems a bit at odds with the low carb theory).