History of Computers
With the complex adding systems that we have today, it can be hard to grasp that peoples were using small stones or other objects as numerical devices from time immemorial. The word calculate itself comes from the Latin calculus, which means small stone. These methods of calculations introduced some elementary kind of abstraction, but people gradually realized that this method did not go far enough to satisfy their increasing needs. To count up to 1000, for example, they would have had to gather a thousand pebbles, which was enormous work.