I'm not sure if this varies from culture to culture, but the general rule at the dinner table seems to be "finish your food". Your parents encouraged (or strictly enforced) it from an early age so it's likely been ingrained into your eating habits like a tumor. As a result, you've probably had numerous meals where you forcibly shoved food down your throat just to clear your plate. The main reason behind this form of parenting is obvious: wasting food is inconsiderate to those who are less fortunate. This is a perfectly acceptable cause, but being who I am, I couldn't help but question it a bit.
