Snapchat CEO Evan Spiegel's hands are shaking as he points to his iPhone.

He's unmistakably nervous, and not in a sweaty, early-Mark Zuckerberg kind of way. There must be a lot on his mind as the young CEO of a company bounding toward a $1 billion valuation — a company that has changed the course of being a teenager in the year 2013. Spiegel brushes off Snapchat’s latest bragging right: the service sees 350 million snaps sent per day. He seems anxious, as if he's about to interview for a job or deliver a commencement speech to his graduating class.

Instead, he tells me about Stories, his team’s latest invention: a rolling compilation of snaps from the last 24 hours that your friends can see. You create your Story as you go about your day by tapping "My Story" above the friends you want to send a snap to. Or, you can tap a new shortcut button in the app's camera screen to instantly post a snap to your Story. But unlike conventional snaps, Stories don't disappear in a puff of ephemeral smoke after you've watched them. You can watch a friend’s (or your own) Story over and over.

Each Story is the sum of all the snaps you’ve added over the last 24 hours, which means its size is always fluctuating. What doesn’t change is that every piece of the Story is less than a day old, so viewing one might be the fastest way to see what a friend's been up to. Snapchat may not look much like Facebook, but with Stories, the company is taking its first steps toward competing with Facebook’s most important product: News Feed.

Behind Stories is a deep understanding, or perhaps loathing, of the way social apps work today. Spiegel claims to have no special knowledge of the way we work as social organisms aside from what he learned as a college student, but has thus far proven himself and his colleagues to be surprisingly thoughtful about our hidden social behaviors and desires. Stories is the next big piece of how Snapchat thinks social media should work, and everybody’s watching. No wonder Spiegel’s a little nervous.