A computer follows a list of instructions and keeps everything as numbers. A computer is built from a few main parts, including a CPU and memory. A program is a list of instructions for a computer. The processor follows instructions that a programmer writes. The CPU can add two numbers. It runs a program by repeating a cycle: fetch an instruction, then execute it. This is a simple picture, because a real computer does far more.

A bit is the smallest piece of computer storage, and it holds a 0 or a 1. Binary is a number system that uses only two digits. A byte is a group of 8 bits. One byte can store one letter or other character, such as A or x. A byte can make 256 different patterns, because each extra bit doubles the number of different patterns that bits can make. Bits are used to store text, numbers, images and video.

Before a program runs, the computer copies it from storage into RAM. RAM is fast memory that holds the programs and files a computer is using right now. RAM loses what it holds when the power is turned off. A bit is stored as electric charge in a chip, but as magnetism on a hard disk. Files that are saved on a computer's drive are still there when the power goes off.

A picture on a computer screen is made of many tiny squares called pixels. A picture on a computer is stored as a long list of numbers. Computers also turn real-world signals such as sound into lists of numbers. Then computers can squeeze pictures and sound into less space. This is called compression. So numbers are what a computer keeps, from a picture to a song.

Words to know

• binary: two-digit system
• compression: squeezing data
• pixels: tiny squares
• RAM: fast memory