Instructions: Given a scientific passage and an answer, generate a question for the given answer.
Input: Passage: When water freezes in a non-crystalline, amorphous form as frost, the water molecules are fixed in place, but not in a regular, repeating pattern. Frost and other amorphous solids are characterized by their unorganized and unpredictable structure. Because ice comes in both amorphous and crystalline forms, there are differences in the physical properties of ice such as its hardness and density. Answer: amorphous
Output:
What kind of solid is characterized by an unorganized and unpredictable structure?