How big do you think a computer is? Computers come in many different sizes. A computer can be big enough to fill a room about the size of a school hall, or small enough to rest on your knees.
The largest computers are called mainframe computers. They are found in the head offices of banks and other large companies. Many of the bills that arrive at your home, such as gas and electricity bills, have been calculated and printed by mainframe computers. Mainframes work very fast, can do lots of different jobs and have very large memories in which they can store programs and data.
Mainframe computers use a great deal of energy and give off a lot of heat. Special air-conditioned, dust-free offices are built to house them.