Without thinking of it, we've probably used piezoelectric materials many times today. If you have a quartz watch, piezoelectricity is what helps it keep time. By definition, piezoelectricity is the appearance of electrical potential across the sides of a crystal , like quartz when stress is applied. Piezoelectricity can be explained in a rather simplified fashion like this

1. Normally, to start off, the charges in piezoelectric crystals are perfectly balanced.

2. The effects of the charges, therefore, cancel each other out, leaving no net charge.

3. If one squeezes the crystal, the charges become massively out of balance. 

4. As these charges are now out of balance, negative charges appear on the crystal face and produces a voltage, which is called Piezoelectricity.                                                                                  Piezoelectricity is used in ultrasound equipment, microphones and watches.