How do barcodes work?

These days you likely never give a second thought to the black and white lines and numbers on the labels of virtually all the goods we ever buy. We are of course speaking about the unassuming barcode, a piece of modern numerical witchcraft that allows a computer to tell the difference between a lemon and a lime. So let's break it down to understand exactly what everything means and why the barcode is a foundation of modern shopping.