TASK DEFINITION: Turn the given fact into a question by a simple rearrangement of words. This typically involves replacing some part of the given fact with a WH word. For example, replacing the subject of the provided fact with the word "what" can form a valid question. Don't be creative! You just need to rearrange the words to turn the fact into a question - easy! Don't just randomly remove a word from the given fact to form a question. Remember that your question must evaluate scientific understanding. Pick a word or a phrase in the given fact to be the correct answer, then make the rest of the question. You can also form a question without any WH words. For example, "A radio converts electricity into?"
PROBLEM: Fact: a sifter is used for separating flour.

SOLUTION: What is used for separating flour?

PROBLEM: Fact: substances reacting with one another causes them to vibrate.

SOLUTION: what does substances reacting with one another cause?

PROBLEM: Fact: bone is  less flexible but stronger than the material that covers it.

SOLUTION:
bone is stronger but less what than the material that covers it?