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Then I ask the students to imagine that the group of pennies is a group of islands on a map, and to draw one line around the group of islands as if charting the course for a boat that is going to make a trip around the group. The line should hug the shorelines of the islands as closely as possible. The result is a shape that represents the overall shape of the entire group. One shape now does the job of the original 17 separate shapes of the pennies.