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Being an archeologist is a lot like being a detective. You spend most of your time trying to find clues, collecting information, and putting pieces together to solve a mystery. Both professions require a variety of tools and draw information from many different scientific disciplines. In fact, the only major difference is that detectives usually can question a witness while archeologists are lucky to find the witness's home! In the hands of a skilled archeologist, however, even a pile of trash can speak volumes. Archeologists must first know where to look for a site. The passage of time and the forces of nature often erase even the sturdiest buildings. Walking an area, archeologists look for special clues that tip them off to human occupation. Some broken pieces of pottery, an old arrowhead, or even a pile of stones can lead them to a site. Before digging, however, they map the surface in detail and sometimes attempt to "see" below the ground with the help of remote sensing techniques like radar. Once digging begins, workers scrape off each level in neat horizontal layers, a painstakingly slow process. Archeologists carefully observe rules of three important concepts: stratigraphy, superposition, and context. Stratigraphy means that the material covering the site was placed there in layers-or strata. Superposition means that the deeper you dig, the older the material gets. While these rules don't give exact ages, they allow scientists to calculate relative time, which is almost as important. Context is also a critical concept, since artifacts only tell a story through the context in which they're found-where they are found and with what they are found. After uncovering artifacts, an archeologist relies on techniques like X rays and chemical and microscopic analysis to determine what materials were used to make the artifacts, how they were made, their age, and their purpose. If enough artifacts are found, the archeologist puts them together into common groups called assemblages. By studying assemblages, the archeologist can then determine what people did in different parts of the site at different times.