Becoming one with CALCULATE()

Hey, it starts out simple and powerful: CALCULATE is the SUMIF you always wished you'd had. It works in pivots. It's the "anything IF." It's amazing, really, how many doors it opens. Of course, CALCULATE is designed to be powerful in ways we can't even IMAGINE in our first day/week/month of using it. You can spend years discovering all the things it can do - and that's a good thing! But sooner or later you're going to hit something with CALCULATE that makes you scratch your head - why is it returning THOSE results?