I reckon I can touch a flat surface with about three different, reproducable levels of pressure (‘resting on’, ‘positive contact’ and ‘pressing hard’, say).
Touchscreens are usually not directly sensitive to pressure level, but almost all of them should be capable of differentiating between these three conditions (even if only by virtue of their different optical effects).
Today’s invention is a way to identify yourself via a touchscreen.
Your new, ten-digit pin code consists of a pattern of different levels of pressure when in contact simultaneously with a screen. This gives a total number of ~59k different codes (ie >>10^4, as commonly used with keypads).
It might take a bit of practice, but once burned into motor memory, this would be super quick and impossible to shoulder-surf.