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.

Fact: rivers can slowly dissolve carbon in sedimentary rock. 
Question: what can slowly dissolve carbon in sedimentary rock?
rivers.

Fact: An object's shape can be changed by removing material. 
Question: What is one way an object's shape can be changed?
removing material.

Fact: ink is used for hiding from sharks by octopuses. 
Question: what do octopuses use ink to hide from?

sharks.