Given scientific fact as input, generate the question from this fact such that it can be answered from the input.
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Question: The seven unique crystal types are defined by faces parts of themselves (parts that intersect at various angles).

Answer: The seven unique crystal types are defined by what parts of themselves (parts that intersect at various angles)?

Question: The nuclear envelope is a double-layered plasma membrane like the cell membrane, although without membrane proteins.

Answer: The nuclear envelope is a double-layered plasma membrane like the cell membrane, although without what?

Question: The high points in a transverse wave are called crests.

Answer:
What are the high points in a transverse wave called?