Sometimes your Finder windows look like this, with a whole bunch of folders flipped open:
Turns out you don’t have to close those folders one by one. You can close them all in just two keystrokes:
⌘-A, then ⌘-⬅
(Command-A, then Command-left arrow)
This works because ⌘-A “selects all” and ⌘-left arrow collapses whatever is selected. In practice it looks like this:
Click somewhere else when you’re done in order to “unselect.” Easy.