Suppose you're strolling through the park one day (say, in the merry, merry month of May) when you spot a phone lying in the grass. Upon closer inspection, it's an iPhone -- and, like all phones should be, it's locked.
Now what? You're a good Samaritan, but how can you reunite the precious with its owner?
Simple: ask Siri. When you invoke Apple's gal Friday and say, "Who does this phone belong to?", you'll see the owner's contact information. This might help you reach out and notify the person before he/she activates Lost Mode and possibly remote-wipes the phone so there's no contact info left.
Now for the flip side: if you're the owner of an iPhone that goes missing, just what information is going to be revealed to someone who uses this tip?
The answer: the full contents of whatever contact you've designated as "me." That might include more information than you'd want a stranger to see, even one just looking to do a good deed.
The solution, then, is to create a new "Lost Phone" contact, put in only your emergency-contact info, then let your iPhone know that that's you. Or, rather, "me." (But, you know, you.)
Once you've created that new contact, tap the Settings app, then Mail, Contacts, Calendars. Scroll down until you find My Info, tap it, then choose your newly created contact -- the one you want to appear if someone asks Siri who owns your phone.
There are other ways to help get your lost phone back, of course, like adding emergency-contact info to your lock screen. And that would probably save someone from bothering with the Siri option.
While we're on the subject of cool Siri tricks, don't forget the best Siri feature you're not using. Think there's a better one? Name it in the comments!