An amaretto sour is a member of the "sour" family of cocktails, which are very simple to make. "Sour" cocktails include only a few ingredients: - a liquor of some sort (in this case amaretto) - sour juice (usually lemon or lime) - a sweetener (like simple syrup, sugar, or honey) I skipped the sweetener in this case, because amaretto is already quite sweet, and I didn't find that it needed extra. Some sours are also made with egg white, which gives the cocktail a thick, creamy foam on the top after shaking.