Instructions: Given a scientific passage and an answer, generate a question for the given answer.
Input: Passage: Tar sands are rocky materials mixed with very thick oil. The tar is too thick to pump, and so tar sands are strip-mined. Hot water and caustic soda are used to separate the oil from the rock. This creates a slurry, which is shaken. The oil floats to the top and is skimmed off. Answer: very thick oil
Output:
Tar sands are rocky materials mixed with what?