"Don't be nosey!" That's what I used to tell my friends when they borrowed my phone. Inevitably, they would swipe through my photos or put a silly comment on my Facebook. But you can stop this kind of intrusion by setting up either a guest mode or privacy mode on your Android phone.
How to use guest mode
Not all Android phones have guest mode capability. Not all Android phones have guest mode capability. Some that do are the and the Nexus. Basically, the mode is on Android phones running 5.0 Lollipop and newer.
Here's the method on the Pixel 2. Go to Settings > Users and accounts > Users > Guest. Once you're there, you can set up a profile by tapping Add new user > OK > OK. Once you have the new user set up, you can switch over to guest mode by extending the notifications menu, tapping the icon of a human head (look for it by the date) and selecting the guest.
To set up a guest mode with the OnePlus 5T, tap the User icon (the person inside of the circle) > Add guest. Now your friend can use your phone without peeking at your stuff.
When your friend is done on either phone, tap User icon > Remove Guest > Remove.
If you let the same person borrow your phone over and over -- and they don't care about their data being on your phone -- you can just leave the guest account and switch to your profile when you tap the User icon. You'll need to sign in with your password, fingerprint or lock pattern to access your stuff. The next time your friend uses your phone, just tap the User icon > Guest User icon > Continue.
Turn on privacy mode
If your phone doesn't have guest mode or runs something older than 5.0 Lollipop, don't despair. Other phones come with a privacy mode that can hide your secrets.
On some phones you can go to Settings > Privacy and Safety > Private mode. Then, toggle the Private switch to On. To choose which files to make private, go to your files, select the files you want to hide, tap More and choose Move to Private. Your files will be found only in your private folder and can't be accessed without your phone's password.
Other Android phones, like the Galaxy 8, do it a little differently. Go to Settings > Lock Screen and Safety > Secure folder. Next, select a lock type that will protect your folder. Tap Add Apps or Add Files to put things in your folder. Now, no one can look at these items without your fingerprint, lock pattern or password.