Preview a file with the space bar

Select a file in the Finder and press Space. A window opens with a full-size preview — a photo, a PDF, a video that plays — without opening the file in any app. Press Space again and it closes.

This is Quick Look, and the arrow keys are what make it worth learning. Select all the photos in a folder, press Space, and ← and → walk through them one by one. Finding the right shot out of two hundred takes seconds. With several items selected there is also the Index Sheet button, or ⌘ + Return, which shows them all as a grid.

It is not a Finder trick either: the same key works on the desktop, on attachments in Mail and Messages, and in plenty of other places where files are listed.