I can't image this, Someone can give a clear illustration?
closed as not a real question by Harry Gindi, Joel David Hamkins, Ryan Budney, José Figueroa-O'Farrill, Theo Johnson-Freyd Mar 9 '10 at 3:43
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This is covered in Chapter 0 (the introductory chapter) of Algebraic Topology by Allen Hatcher.
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