In some places, sand dunes protect shorelines from the onslaught of ocean waves. In other places, the dunes themselves are on the move, and threaten human structures.Host Flora Lichtman talks with mechanical engineer Nathalie Vriend, who studies the structure of sand dunes, about what makes a heap of sand a dune, and what scientists still hope to learn about sand.Guest: Dr. Nathalie Vriend is an associate professor in mechanical engineering and leader of the Granular Flow Laboratory at the University of Colorado in Boulder.Transcripts for each episode are available within 1-3 days at sciencefriday.com. Subscribe to this podcast. Plus, to stay updated on all things science, sign up for Science Friday's newsletters.

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“What did the council decide on item 42?” — answered with the speaker and the moment.