Let’s say you want to watch a movie. Downloading it would be a matter of minutes. When ordering it online, the shipment might take a week to arrive. Imagine now you want to watch ten movies. Downloading them might take a an hour or two now. But the shipment would still arrive in one week. Now what about twenty movies? Or maybe even a hundred?

When analyzing the complexity of an algorithm, it is important to think about the scalability of a program. It tells you how the runtime of a program depends on the number of elements processed. In the initial example, the time needed to download the movies depends on the amount. Computer scientists would say: The runtime scales with n, where n stands for the number of movies. When having them shipped, the waiting time doesn’t depend on this number, which means it doesn't scale.