Given a scientific passage and an answer, generate a question for the given answer.
Q: Passage: When the wind deposits sand, it forms small hills. These hills are called sand dunes ( Figure below ). For sand dunes to form, there must be plenty of sand and wind. Sand dunes are found mainly in deserts and on beaches. Answer: sand dunes
A:
When the wind deposits sand, it forms small hills, what are these hills called?