Inside the Central Processing Unit (CPU) there is a 'clock' - the CPU clock 'ticks' by switching from high to low and then back again in a very precise time:
Every time the clock pulses an instruction is carried out: Clock speed is measured in pulses per second. Since "1 pulse per second" is also known as 1 Hertz computer speed is often specified in 'Gigahertz' (GHz). The CPU clock speed on personal computers runs from less than 1GHz to about 3.8GHz
This means that the machine can carry out a billon instructions per second when running at 1GHz.
If it runs at 3.8GHz then three point eight billon instructions per second could be carried out.
Although it may seem that the computer is doing several things at once, in reality the computer is just doing one thing at a time.
Challenge see if you can find out one extra fact on this topic that we haven't already told you
Click on this link: Clock Speed