Piezoelectric effect

The piezoelectric effect is the ability of some crystals and ceramics to produce a voltage when squeezed, and to change shape slightly when a voltage is applied. It works because the material's internal electric charges shift when the crystal structure is deformed. It is used in sensors, buzzers, and ultrasonic devices.

Example: The click spark in a push-button gas grill lighter comes from a spring-loaded hammer striking a piezoelectric crystal.