Clocks in computers are formed by applying electric current to certain crystals. Quartz is often used for this purpose. When electricity is applied to a quartz crystal, it expands and contracts rhythmically as the electrons travel through it. This rhythmical change in the shape of the crystal can be used to cause changes in the voltage, thereby emitting a series of on/off pulses at a very high frequency. When you buy a new Pentium that has a 166 MHz clock, you are buying a computer with a clock that pulses 166 million times per second.