Updated June 10th, 2019.
You know you can grab any edge of a window and drag to resize the window. Everyone knows that. But… if you hold the Option key while you do it, the window grows in the opposite direction too!
For example, if you’re grabbing a window’s right edge and dragging it to the right, the window grows to the right… unless you’re holding the Option key. In that case, the window also grows to the left! You have to try it to believe it. Works with up/down too. Try it!
It also works in Pages and Keynote with graphic objects. It’s very handy for when you want to grow something from the center rather than from the edge. For example, if you have an oval exactly where you want it, but you want it bigger, if you resize it normally the center will move. But, if you resize it while holding the Option key, the center stays put. Sometimes that’s exactly what you want.