OK, so you've undoubtedly heard of Snapchat, an app that allows you to send a photo or video from your phone and determine how long the person on the other end can see the image until it self-destructs.

But what you probably didn't know is that a lot of images from Snapchat are regularly posted to revenge porn sites, called "snap porn."Snapchat may not be the #1 app used for sexting but that's not to say it isn't the principal appeal of the app for many: Users think their snaps will disappear and they are wrong.

"It's a key way teens are hiding their nude pictures from their parents," said Lewis.

Same deal, but this time with a calculator icon posing as something it isn't.

Sedgrid Lewis, online safety expert, notes that these apps look like a normal calculator app but when teens push a button within the app they can hide all inappropriate pictures.

Not for nothing, Snapchat last year published a “Snapchat Safety Center” reminding kids that nude pictures were not allowed.

“Don’t use Snapchat for any illegal shenanigans and if you’re under 18 or are Snapping with someone who might be: Keep your clothes on! The reality is, Snapchat is likely on your kid's phone.

The best control you have (besides taking the phone away) is to just have a frank heart-to-heart about how there is no such thing as texts or photos that disappear and this is some down-and-dirty stuff that can come back to haunt them.

Like Snapchat, Burn Note is a messaging app that erases messages after a set period of time.

But since there are no authentication requirements, sexual predators can contact minors and minors can hook up with adults -- and of course there is the sexting, notes For Every 10. Ki K is an instant messaging app that lets users exchange videos, photos and sketches. Ki K does not offer any parental controls and there is no way of authenticating users, thus making it easy for sexual predators to use the app to interact with minors. It's also the app du jour for sending a bomb threat to your school. Elizabeth Long, an Atlanta teenager who was encouraged on Yik Yak to try harder to kill herself after her attempted suicide failed, led a drive to shut the app down.

She wrote, "With the shield of anonymity, users [of Yik Yak] have zero accountability for their posts, and can openly spread rumors, call classmates hurtful names, send threats, or even tell someone to kill themselves -- and all of these things are happening." 12. This is one of the most popular social networking sites that is almost exclusively used by kids.

Not everything online is evil, nor does danger lurk behind every new app that comes to market. Kids can hide any app they don’t want you to see, Teen Safe says.

But keeping up with your teens' and preteens' online activities is much like trying to nail jelly to the barn door -- frustrating, futile and something bound to make you feel inept. Such is the case with Audio Manager, an app that has nothing to do with managing your teen's music files or controlling the volume on his smartphone and everything to do with him hiding things like nude photos from you. When you press and hold the Audio Manager app, a lock screen is revealed -- behind which users can hide messages, photos, videos, and other apps.

Many users ask for personal data upfront, including location, age, and gender [ASL], something kids might supply (not realizing they don't have to).