The way it works

You know how Gmail categorizes emails? Some of them get labeled as important, and I don't know what the other classifications are, because I switched my setup to having the filtered tabs and I don't think it does the labeling anymore. Huh. Go figure. Anyway...

How does Gmail know which emails are actually important enough to get that label? Algorithms based on key words, people the email was sent to, etc.? Nope. That's not it at all. The answer is much more interesting than that:

So that's how they know. Magic sauce. I think I need to get my hands on some of that.