Write a correct answer to the given question based on its associated fact. Make sure that your answer is contained in the associated fact. Don't be creative and introduce any new word that is not mentioned in the associated fact! Remember that the associated fact has been rearranged to form the question. So, the correct answer words must lie within the associated fact. The correct answer can be a word, phrase, or even a sentence.
--------
Question: Fact: ancestors of hagfish probably had backbones. 
Question: What did ancestors of hagfish probably have?

Answer: backbone.


Question: Fact: Condoms make sex safe if they're not used incorrectly. 
Question: When aren't condoms safe?

Answer: When they're used incorrectly.


Question: Fact: the type of material through which mechanical waves pass change the speed at which they travel. 
Question: The type of material through which mechanical waves pass through changes what?

Answer:
their speed.