The touchscreen locates your finger on the grid by measuring how much the charge falls between two intersecting electrodes. This process is referred to as “mutual capacity”.
The problem is that drops of sweat or raindrops can also reduce the charge by providing another channel between the electrodes. Fortunately, the touch screen engineers solved the water problem by resorting to a different type of touch detection called “self-capacitance.”
Instead of measuring the charge between electrode pairs, the touchscreen measures the charge increase between a single electrode on the screen and the floor you are on. Since the water droplets are not grounded, the firmware of the phone can better ignore them.
However, it is not completely successful because it does not work with multipoint touch operations such as pinch, zoom, and so on.