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: Cold fronts cause lightning. 
Question: Cold fronts cause what?

Answer: lightning.


Question: Fact: A person's elbow straightens when muscles antagonistic to biceps contract. 
Question: A person's elbow straightens when muscles antagonistic to what contract?

Answer: biceps.


Question: Fact: a delta is formed by erosion. 
Question: A delta is formed by?

Answer:
erosion.