An allotrope is when two physical forms of the same element or compound exist at the same temperature, even though they have identical molecules. The canonical example of an allotrope is carbon. Graphite is one allotrope of carbon, and diamond is another (recently scientists have been playing around with other allotropes of carbon, such as buckminsterfullerenes, also known as buckyballs). They are both made of carbon, but in different atomic configurations; for example, graphite has a planar structure (and hence, it makes a good non-liquid lubricant), while diamond has a tetrahedral structure (which makes it very hard and strong).

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