About 10,000 chemicals are added to Americans' food to make it taste better or look better, to thicken it, preserve it, or otherwise improve it. That's an awful lot of chemicals for the federal Food and Drug Administration to monitor. How do they do it? It turns out that, often, they don't. According to the authors of a study published in JAMA Internal Medicine this month, the agency doesn't even know what all the food additive chemicals are.