Given a scientific question and its correct answer, generate supporting facts for the answer. This supporting fact can be an explanation for the answer to the given question.
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Question: Question: Which kind of irrigation uses much less water than other methods? Answer: drip

Answer: Drip irrigation uses much less water than other methods.

Question: Question: What causes blue litmus to turn red? Answer: acid

Answer: Acids change the color of certain acid-base indicators. Two common indicators are litmus and phenolphthalein. Blue litmus turns red in the presence of an acid, while phenolphthalein turns colorless.

Question: Question: A crater can usually be found on the top of what kind of volcanoes? Answer: composite

Answer:
Composite volcanoes usually have craters on the top. Why are the craters sometimes "U" or horseshoe-shaped?.