Perfect for fishing line and car hoods, the double fisherman's knot (shown above) is very difficult to untie. While not so great for your buddy's shoes or a magician's hands, the double fisherman's knot is easy to create and guarantees a very strong hold.
The Impossible Knot [MetaCafe via Hacker Zen]
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The Impossible Knot [MetaCafe via Hacker Zen]
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