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A very important concept in Buddhism is that of impermanence, being one of the
Three Marks of Existence (for more information, check out the link). Impermanence is more than just a nice concept, though, as evidenced by the practice of creating Sand Mandalas. The mandalas themselves are absolutely incredible... check out some of the
pictures for yourself. Go through all of the pages to see the construction from beginning to end, and the subsequent reminder of impermanence. The
BBC has more information on the importance of the mandala's ritual creation and destruction.