What does piezoelectricity in crystals cause them to do?

Piezoelectricity in crystals refers to the ability of certain materials to generate an electric charge in response to mechanical stress. This property causes the crystals to vibrate when an electrical current is applied, resulting in oscillations at a specific frequency. This phenomenon is utilized in various applications, such as in quartz crystal watches, where the oscillation frequency helps keep accurate time.

The other options, while they involve physical changes or reactions, do not accurately describe the effect of piezoelectricity. For instance, emitting light when heated relates to thermoluminescence, changing color with temperature pertains to thermochromism, and absorbing moisture from the air describes hygroscopic materials. These processes do not have a direct relationship with the mechanical and electrical interactions that occur with piezoelectric materials.