We’ve all been hearing about the cool app Snapchat, which allows people to send pictures and videos that only last a few seconds before disappearing. Because of the disappearing thing, the worry that I keep reading about is that teens will use it for sexting, figuring that it’s no problem if they take sexy pictures or videos, because they won’t last.

Now I worry about sexting as much as anybody else (although a 2011 study suggests that not that many teenagers actually do it). Sexting can glamorize and normalize sex in a way that might cause some teenagers to start having sex earlier, or in unhealthy ways. And legally, sexting can possibly be considered sexual harassment–or worse, distributing pornography.

But that’s not my biggest worry about Snapchat.

Here’s the thing: the pictures and videos don’t necessarily disappear. The way the app is set up, someone can take a screen shot. While theoretically the sender should be notified if a screen shot gets taken, it only took me a couple of minutes on Google to find hacks that would allow me to take screen shots or save video without the sender knowing.

I worry about Snapchat because it creates the illusion that something can disappear from social media–and that is really dangerous. The biggest two lessons that youth need to learn–actually, that everybody needs to learn–about social media are: 1. Nothing is private and 2. Anything you do on social media can last forever.

We keep hearing about people losing their chances at jobs or school admission because their prospective employers or admissions officers go online and check them out. They look at publicly available stuff–tweets, Facebook posts, etc–and they don’t like what they find. We are all doing it, actually: making judgments about people based on what they post.

The other day, my 15-year-old daughter got mad that I looked at her blog. She saw it as an invasion of her privacy. Um … sweetie? The entire world can look at it. But I’m asking you not to, she said. It doesn’t work that way, hon. You don’t pick who looks at what you put online.

We just don’t think about this stuff as we are posting and tweeting and sending photos and texts. A couple of days ago my 20-year-old son took a screen shot of a text conversation we’d had and posted it on Facebook. I didn’t really mind, but it was definitely one of those note-to-self moments. Anything we email or post or text or send can go anywhere. Literally.

This is a really important, and yet really hard, lesson to learn. And apps like Snapchat make it harder by making it seem like it’s possible that social media could actually be ephemeral or private.

I love social media, and think that it has tremendous potential to help us in so many ways. But it isn’t ephemeral or private. At all. The sooner everyone learns and lives that, the better.

Claire McCarthy is a primary care physician and the medical director of Boston Children’s Hospital’s Martha Eliot Health Center.  She blogs at Thriving, the Boston Children’s Hospital blog, Vector, the Boston Children’s Hospital science and clinical innovation blog, and MD Mama at Boston.com, where this article originally appeared.