If you ever need to hand your phone to someone but don't want them going through it, Samsung has a great feature called "pin app." It essentially turns your phone into a kiosk, only letting that one app be run unless the phone is unlocked.
Click the three vertical dots (the app switcher) and select the application you want to pin. Tap the icon of the app in the switcher, and a label that says "pin this app" will appear.
When you click it, only that application will run. If you want to unlock it, hold down the back and recent buttons and type in your passcode. This works with almost all applications except the native dialer.
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