Instructions: Given a scientific passage and an answer, generate a question for the given answer.
Input: Passage: Grain boundaries. As a polycrystalline material solidifies, grains with irregular shapes form. The interfaces between grains constitute grain boundaries. (Squares represent unit cells within grains. Answer: boundaries
Output:
As a polycrystalline material solidifies, grains with irregular shapes form. the interfaces between grains constitute grain what?