human
How do you know if a sand dune is moving or not?
assistant
Sand dunes are made up of loose grains of sand. If the wind blows over them, it can cause the grains to move and shift around. This causes the shape of the dune to change, which is called 'active'. If the wind isn't strong enough to move the sand, the dune is said to be 'passive' and will stay in its current shape.