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Aside from ancient buildings, in sheer bulk the largest part of the archaeological record is made up of stone tools and pottery fragments (shards). Stone tools are the earliest known artifacts, having been first used more than two million years ago, and they have remained in use to the present day. When a chunk of fine-grain stone is struck with sufficient force at the proper angle with another rock or with a wood or bone baton, a shock wave will pass through the stone and detach a flake of the desired size and shape. In analyzing ancient stone tools, many archaeologists have mastered the skills needed to make stone tools themselves. Few things are sharper than a fragment struck from fine-grain flint or from obsidian (volcanic glass). Obsidian is so fine grained that flakes of it can have edges only about twenty molecules thick – hundreds of times thinner than steel tools. Through experimentation, some archaeologists are able to produce copies of almost every stone tool type used in antiquity. A common research strategy is to make flint tools, use them to cut up meat, saw wood, clean hides, bore holes, etc, and then compare the resulting wear traces with the marks found on ancient artifacts. Sometimes electron-scanning microscopes are used to study minute variations in these use marks. Some rough correspondence can be found between the types of uses and the characteristics of wear marks, but there are many ambiguities. Ethnographic data from people who still use these tools, like one study of how the IKung hunter-gatherers use different styles of stone spear points to identify their different social groupings, indicate that even crude-looking stone tools may reflect a great deal of the social and economic structure. Ceramics were in use much later than the first stone tools (appearing in quantity in many places about 10,000 years ago), but they were used in such massive quantities in antiquity that, for many archaeologists, work life consists mainly of the slow sorting and analyzing of pottery fragments. Ceramic pots were first made by hand and dried in the sun or in low temperature kilns, a process that did not produce a very durable material. But in many areas of Africa, Asia and Europe high-temperature kilns produced pottery that is nearly a form of glass, and fragments of these pots survive even when the pottery is broken. Ceramics form such a large part of archaeologists' lives because ceramics express so much about the people who made them. Pots are direct indicators of function in that they show how diets and economies changed over time. Archaeologists have documented how pottery in the American Southwest changed in prehistoric times as a diet developed that included boiled seeds of various native plants, and pottery was developed to withstand the heat and mechanical stresses of the boiling process. Ceramics are almost always analyzed on the basis of their style. This idea of style is hard to define, but changing styles are the basis on which archaeologists date much of the archaeological record. But for many archaeologists, ceramic styles are more than just convenient devices of dating. For many archaeologists, stylistic decoration of artifacts is the primary means by which one can enter the cognitive world of the ancients. Societies throughout history have invested their objects with styles that have profound and complex meanings and effects. In the case of the Maya and every other early civilization, rulers used particular symbols and styles as mechanisms through which they portrayed, communicated, and implemented their power. In all societies, styles fix social meaning and are powerful ways in which social groups define and construct their culture. Styles of objects, language, and personal behavior identify people in terms of gender, age group, ethnic group, socioeconomic class, and in many other important ways. Some researchers, for example, have argued that a particular kind of pottery, called Ramey incised (which is incised with figures of eyes, fish, arrows, and abstract objects and was used by the people in the area of present-day Missouri and Illinois at about A.D 900), was primarily used to distribute food but was also used to communicate the idea that the society's elite, for whom the pots were made, were mediators of cosmic forces.