Open Question: Are hammocks adjustable?

Hi, I'm thinking of buying a "free-standing hammock" for my son for his birthday. He wants to set it up between two trees nearby, but all the hammocks I see are all 13 feet long, and these trees are not exactly that size. Does anyone know if these hammocks are adjustable? (In other words, is there any way I can change the "length" of the hammock somehow?) He does not want the hammock stand. Any info. would be greatly apprecaited!! Thanks.

Source:
http://answers.yahoo.com/question/index?qid=20110604171805AATwyxk

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