Most modern phone screens use capacitive touch sensing. Beneath the glass is a transparent pattern of conductors that creates and measures tiny electric effects across the screen. Your body conducts electricity. When your finger approaches or touches the glass, it changes the local electric field and capacitance measured by the sensor grid. Electronics detect where that change occurred and software converts it into a tap, swipe or gesture. The screen can repeat this many times each second. The system is measuring changes in electrical behaviour, not pressure from your finger.

The useful part of this physics is that it helps people predict and design real things. Engineers often begin with the same simplified model used in school, test where it works, then add more detail when measurements show that the real situation is more complicated.

At the microscopic level, the important particles are electrons. Moving electrons from one material to another leaves an imbalance of charge, and charged objects create electric fields around them. Those fields can attract or repel other charges without contact. In electronic sensors, very small changes in charge or capacitance can therefore be turned into a measurable electrical signal.