John Baez on the topic of constructing a topological space whose first homotopy group is the rationals Q cracks me up: Mathematicians call this general type of space a "telescope". An ordinary hand telescope - the kind that pirates use - is built from cylinders of metal that fit into each other: A mathematician's telescope is similar - but it's built from infinitely many cylinders, and you can't collapse it, because they're attached to each other in a complicated way. This makes it really easy to spot a mathematician in a roomful of pirates. (emphasis mine)