Lock your phone to a single app with Lollipop’s screen pinning

Here’s the scenario: You hand your phone to your three-year-old to keep them amused in the back seat of the car. When you get your phone back 20 minutes later, though, you realize that your kid has deleted half your apps, texted your ex from college, and wrote the Great American Novel.
Not good. Except for that last part. That’s pretty cool. You have a smart kid.
But you could have averted this digital disaster if you used Android 5.0 Lollipop’s screen pinning feature. This new addition makes it more difficult to switch from one app to another—and accidentally wreak havoc on someone’s phone. Here’s how to set it up.
Open the Settings app, tap Security, then tap Screen pinning—it’s located under the “Advanced” heading. Next, toggle the screen pinning toggle to the “on” position.

Turning on screen pinning is just a few taps away.

To use screen pinning, tap the Overview button (the square button in the lower right corner of your screen), and swipe up and down until you find the app you want to pin. Once you find the app you want to pin, tap the Pin button—it’s a blue-green button with a white pushpin on it.

Tap the pushpin icon to pin that app to your screen.

When you’re ready to turn off screen pinning and move on to another app, hold down the Back (triangle) and Overview buttons at the same time for a second or two, until your phone tells you that it has unpinned the screen in question. There's even an option to require your security pin/password if you like. So now you can hand your phone over to your kid with a little less anxiety.