Take a picture. Doodle. Text. Put it all together and add some spy-style intrigue.

Welcome to Snapchat, the latest app to be lauded as the "next big thing."

The free app, popular among teens, lets users take a picture and then add a message or doodle on top of the image. Before sending to a friend, the user specifies how long the photo will be visible -- up to 10 seconds -- before disappearing from the phone of both the sender and the recipient.

If a recipient takes a screenshot before the photo or video messages expire, Snapchat notifies the sender.

In mid-December, developers of the year-old app said that more than 50 million "snaps" are shared every day. It's among the top five most popular free apps for iPhone this week, just behind YouTube.

Snapchat's developers have said that the app was inspired by the belief "in sharing authentic moments with friends," going beyond photos of fancy dinners and luxurious vacations that people post elsewhere on social media.

The hype and popularity were enough that even Facebook jumped on board, releasing its copycat app, Poke, in late December. That app isn't yet available for Android, and it doesn't rank among popular free downloads for iPhone.

Experts have raised questions about both apps' possible use for "sexting" and whether the images are truly deleted.

Snapchat says it doesn't keep the images. Facebook says it stores them for a couple of days, in case someone reports abuse.

So snap away. But maybe not with abandon.