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 can think about static electricity like a teeter-totter.
Sent 2: Things are fine as long as they are balanced.
Sent 3: The same can be said about electrons.
Sent 4: Too many or too few can be shocking!
Sent 5: Static electricity is all about that balance.
Sent 6: Charges can build up through friction.
Sent 7: Some materials freely give away electrons.
Sent 8: Other items easily gain electrons.
Sent 9: Have you ever rubbed your feet on a wool mat or carpet?
Sent 10: Rubber soled shoes readily accept electrons.
Sent 11: The wool carpet easily gives up electrons to the shoes.
Sent 12: This combination provides for a large transfer of electrons.
Sent 13: This buildup of electric charges is known as static electricity.
Sent 14: Positive charges build up on the mat and negative charges build up on you.
Sent 15: What happens when an object becomes charged?
Sent 16: A material is likely to remain charged until it touches another object.
Sent 17: Sometimes it will discharge if you just come close to another object.
Sent 18: What comes next may be shocking. 
Question: What happens when an object becomes charged?.
16, 17.