... The food you're consuming is bite-sized.

This rule can be attributed to the fact that, if you can eat something in one bite, all of the calories are broken down in your mouth before your brain can identify that you're eating. And because your brain is unaware of this consumption, it cannot send the signals to your stomach that would normally alert the stomach to extract and store calories from food.

This rule can generally be measured by comparing the size of your snack to the size of your hand (and obviously fluctuates from person to person). If the food you're consuming is smaller than the palm of your hand, it's calorie-free! Larger than that, and I'm sorry to tell you that it does contain calories. It is important to note though, that even though the size-of-snack-to-hand ratio for various sweet items such as cookies and donuts may sometimes be questionable, these foods will always fall under the heading of "calorie-free" when applied in these circumstances.