You probably know that Apple's new Face ID feature will unlock the iPhone X and secure mobile payments with Apple Pay. You might even know that the front-facing camera behind Face ID can apply portrait mode to selfies and also breathe life into animated emojis, or "animoji."

But did you know that Face ID knows when you're looking at the screen, and then makes the iPhone X respond?

Now playing: Watch this: iPhone X's hidden security trick

According to Apple's website, when you're looking at the screen, Face ID will:

Reveal notification and messages for your eyes only



Keep the screen lit when you're reading (don't you hate when it dims before you're ready?)



Lower the volume of an alarm or ringer, because you can hear it, already, I mean, you're right there



Apple isn't the first to keep the screen lit when you're reading; an optional Samsung feature called Smart Stay has done the same since the Galaxy S3 in 2012. Apple's Face ID takes it a step further by linking notifications and ringer volume to your eyeballs (hopefully this won't drain your battery too much in the process).

Now playing: Watch this: See Apple's new animojis in action

This subtler part of Face ID is the type of context-aware AI that devicemakers have been struggling to make people care about. For example, the HTC U series has included AI meant to learn your behavior and surface info like notifications for things you care about most.

Huawei's Honor Magic purported to use the phone's sensors to know who you were, what you were doing and what you wanted next. This kind of background AI so far hasn't caught on, and frankly works with varying levels of success, but it's also something that could quietly become a mainstay for phones in the not-too-distant future.

Apple's least-flashy use for Face ID may be its most convenient for phone owners, because in order to benefit from it, you won't have to do anything at all.

Read next: Apple's Face ID challenge is getting you to trust it

9 things Alexa can't yet do: Our wishlist of features Amazon needs to add to its smart assistant.

Nintendo Switch hands-on preview: Nintendo's new games console wants to be your only one, bridging handheld and TV.