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: Hydrogen bonds cause H20 to expand when it freezes. 
Question: What causes H20 to expand when frozen?

Answer: hydrogen bonds.


Question: Fact: human power causes a bike to move. 
Question: What causes a bike to move?

Answer: Human power.


Question: Fact: animals cannot make their own food. 
Question: What cannot make their own food?

Answer:
animals.