Double tapping mobile's screen: a quick way to make text bigger

Many smartphone users have yet to discover the double tap, the relief for old eyes. Text can be hard to read on tiny mobile screens as eyesight worsens. Hundreds of millions of users know how to spread two fingers apart on the touch-screen to zoom in and make it bigger.
However, an easier option is to just quickly double tap the mobile screen with a single finger to indicate an area that should be enlarged. Most smartphone brands support the function, which can be reversed with a second double tap.