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You are given a paragraph, with numbered sentences, and a question. Write the numbers of the sentences needed to answer the given question. Separate multiple sentence numbers with a comma (", "). E.g. "1, 2" would be the correct response if Sent 1 and Sent 2 are needed to answer the given question (do not include sentences that contain no information that is necessary to answer the question). Sort the sentence numbers in an increasing order (i.e., '1, 2', instead of '2, 1'). Your answer should not contain anything than the sentence numbers separated by a comma (e.g. '1, 2, 3, 4'). The given question requires more than one sentence to answer. So, your answer must contain at least one comma (", ").

Paragraph- Sent 1: You know that friction also causes heat.
Sent 2: Think about when you rub your hands together.
Sent 3: It is friction that makes them warm.
Sent 4: But why does this happen?
Sent 5: Friction causes the molecules on rubbing surfaces to move faster.
Sent 6: Faster moving particles have more heat energy.
Sent 7: Heat from friction can be useful.
Sent 8: Can you think of other places where you might find friction?
Sent 9: Friction also lets you light a match (see Figure 1.10).
Sent 10: Heat from friction can also cause problems.
Sent 11: It can cause a car to overheat.
Sent 12: To reduce friction, oil is added to the engine.
Sent 13: Oil coats the surfaces of moving parts.
Sent 14: This coating of oil makes them slippery.
Sent 15: When things are slippery there is less friction.
Sent 16: Have you ever seen a sign that says, slippery when wet?
Sent 17: This too has to do with friction.
Sent 18: Water, like oil, can reduce friction.
Sent 19: The wet surface may allow your shoes to slide more easily. 
Question: How does oil reduce friction between moving car parts?.
Output:
13, 14, 15.