How Computers Work is a free online explainer from mousecalls that takes pupils inside the machine, from a single switch to a working, programmable computer. It is completely hands-on: every idea is something you click, flip, build or run. There is no login, no download and nothing is uploaded — it runs entirely in the browser on Chromebooks, tablets and computers. It is built for Key Stage 3 Computing and stretches upper Key Stage 2. It is made up of six connected rooms, where each room reuses the ideas from the room before.
Deep down a computer is just millions of tiny switches. Each switch is a bit and can only be off (0) or on (1). Pupils flip switches and light a bulb, then add more switches to see how each extra switch doubles the number of patterns — one switch gives 2 patterns, eight switches (a byte) give 256. This explains bits, bytes and powers of two.
Pupils build numbers using place-value bits (128, 64, 32, 16, 8, 4, 2, 1), convert between binary and decimal both ways, watch a binary odometer count up to see the pattern, and do binary addition with the carry rippling from column to column. This covers representing numbers in binary and simple binary arithmetic.
Pupils see that text, images and sound are all stored as binary. They type a message and watch each letter become its ASCII code and 8-bit binary, paint an 8×8 picture that is linked two ways to its bytes, mix a colour from red, green and blue values (each one byte), and sample a sound wave into a list of numbers. This covers data representation.
Pupils explore the logic gates AND, OR, NOT, XOR and NAND, toggling inputs and reading a live truth table, then wire gates together to build a half-adder and a full-adder — the very circuit that adds the binary numbers from Room 2. This covers Boolean logic and truth tables.
Pupils run a tiny program on a visual processor with a Program Counter, Instruction Register, Accumulator, ALU, RAM and an output. They step through the fetch-decode-execute cycle, watch data travel along the bus, and edit real programs using a small instruction set (LOAD, STORE, ADD, SUB, JUMP, JZERO, JPOS, OUT, HALT). This covers how instructions are stored and executed.
Pupils see how the CPU, memory (RAM), storage, input and output connect along buses, trace journeys such as pressing a key or saving a file, and finish with a capstone that runs a whole calculation through every idea at once. This covers the hardware components of a computer system and how they communicate.
How Computers Work suits KS3 Computing lessons and upper KS2, works on a projector or on pupils' own devices, and is free with no account. No pupil data is collected and nothing is uploaded. More free classroom apps are at mousecalls.com/apps.