This tip has been floating around the Web for a while, but not everyone has caught on—if you're using a Google app such as Chrome or Maps while holding your phone or tablet with one hand, you can double-tap then move your finger up or down to zoom out or in.
Keep your finger or thumb down on the second tap to activate the feature, which makes it much easier to get your bearings on a map if you don't have both hands free and can't use the regular two-fingered pinch-to-zoom functionality. With the rest of your hand behind your phone/tablet of choice, your thumb can now zoom as well as select.
It works in Google Maps, Gmail, Photos and Chrome (and possibly several others) on Android, and in Google Maps on iOS. Double-tapping in Chrome and Gmail for iOS selects text, so you can't use the shortcut for zooming. If you know of any other apps where the trick works, leave us a note below.