In approximately 8000 BC, the Mesopotamians began using clay tokens to count their agricultural and manufactured goods. Later they began placing these tokens inside large, hollow clay containers (bulla, or globular envelopes) which were then sealed. The quantity of tokens in each container came to be expressed by impressing, on the container's surface, one picture for each instance of the token inside. They next dispensed with the tokens, relying solely on symbols for the tokens, drawn on clay surfaces. To avoid making a picture for each instance of the same object (for example: 100 pictures of a hat to represent 100 hats), they 'counted' the objects by using various small marks. In this way the Sumerians added "a system for enumerating objects to their incipient system of symbols" (Writing - Wikipedia, the free encyclopedia)