We are what we eat

I have seen children come into a restaurant really fussing and crying, and after eating, they were cheerful and laughing.

http://www.sciencedaily.com/releases/2008/06/080605150908.htm

ScienceDaily (Jun. 11, 2008) — New research by scientists at the University of Cambridge suggests that the neurotransmitter serotonin, which acts as a chemical messenger between nerve cells, plays a critical role in regulating emotions such as aggression during social decision-making.
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Their findings highlight why some of us may become combative or aggressive when we haven't eaten. The essential amino acid necessary for the body to create serotonin can only be obtained through diet. Therefore, our serotonin levels naturally decline when we don't eat, an effect the researchers took advantage of in their experimental technique.
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The only way to get the raw material for serotonin (tryptophan) is through the diet. Therefore, serotonin levels are lower when you haven't eaten, an effect that the researchers take advantage of in their experimental technique. Eating tryptophan rich foods like poultry (chicken soup) and chocolate can boost serotonin levels - some have speculated that this is why these are "feel good" foods.