Finding an app preventing a Mac from sleeping

My not-an-iMac wasn’t going to sleep, despite the power settings being properly configured under the Control Panel. Activity Monitor to the rescue; right-click the column names row in the interface and add “Preventing Sleep” to find the culprit:

In this case, Firefox had a tab open with video, and Mail was running some interminable search. Closing the offending tab, and quitting Mail, allowed the machine to nap when it wasn’t being used.