It seems like iPhone storage is always at a premium, especially if you have an entry-level model with just 16GB -- an economic necessity for many an iPhone owner. It's the rare person who doesn't know the pain of trying to download a new app or snap a few more photos and seeing the dreaded "not enough storage" message.
You've probably heard all the usual "free up space" tips, but I'm willing to bet you haven't heard this one. It's an honest-to-goodness magic trick, one that may fill you with joy while simultaneously making you a little mad at Apple.
Your iPhone doesn't need to be jailbroken, and this doesn't void your warranty or involve any kind of risk (though see update below). Try it, and see what happens.
Update, Aug. 7, 2017: A couple readers reported that while this trick worked for them in the past, recent attempts failed. In fact, one user was able to complete the movie-rental process, even though iOS reported he ostensibly didn't have enough space for it. Your mileage may vary, but the risk here is you could end up getting charged for a movie. But if you reach out to Apple immediately after and explain that it was an accidental rental, you should be able to get a refund.
Because I currently have "too much" space free on my iPhone, I wasn't able to run another test myself. But when I asked Twitter followers to share their results, at least half a dozen reported success. As with nearly anything involving operating-system tweaks, your mileage may vary.
Step 1: Tap the Settings app and then tap General > About to see how much storage is available. Note that as the "before" number.
Step 2: Open the iTunes Store app, head to movies, and find any really long film. "The Lord of the Rings: The Two Towers" is a good choice at 6.83GB. (You just need to find a movie that requires more space to download than you have available on your phone.) Tap the Rent button twice, keeping in mind you won't actually end up renting it (and therefore won't be charged anything).
Step 3: Once you see the message that there's not enough space available to download your rental, tap Settings. Then navigate back to General > About and you should see a couple hundred megabytes' worth of additional available storage -- possibly even more.
Step 4: Repeat the process! (That is, go back to the iTunes Store app and try renting the movie again.) Each time, you should wind up with another chunk of freed-up storage. Keep going until you either have too much free space, meaning you'd actually be able to download the movie, or the available space stops rising.
What. The. Heck? It's not immediately clear why this works, but it does. I just tested it on an iPhone 5C running iOS 9.1, for the record, that had 4GB of available space. Result after the first attempted rental: 4.4GB.
Alas, that was as far as I got. Repeated attempts yielded no more space. But if you check the (now old) Reddit thread devoted to this topic, you'll see that many users tacked on hundreds of megabytes numerous times, and some were able to reclaim gigabytes of space. The best news of all: There doesn't appear to be a downside to this. It's not like iOS is secretly wiping out your photos or something.
Instead, it's probably clearing out caches that shouldn't be full in the first place -- suggesting this is a storage issue (if not outright bug) Apple should investigate.
Let's crowdsource it a little further. Once you've tried the trick, hit the comments with your results!