Now, let's talk about why "built" cocktails like the Old Fashioned -- i.e. cocktails you can make right in the cup, not in a shaker or mixing glass -- are ideal for parties. First, they're just practical. When you eliminate tools, you eliminate mess. Second, these are the least diluted, warmest cocktails. That means you won't be cycling through tons and tons of ice, and there's generally less added water to control (finicky dilution is often the culprit in a badly mixed drink).