Took me about an hour I'd say to finish all the ones on the first link. There's 30.
The trick once they start getting difficult (ie: You can't just draw mostly straight lines), is to look for the "floater". Most of the starting points will be locked in some way. They're on an edge, or are next to (or diagonal to) another start point that is. Hence, you can't get around those. A floater is one where it's all by itself. Not on an edge, and not next to another start point. Think of it as tying a knot and that's the bit you have to wrap around. The solution will always require you to figure out which one of the other colors has to travel around that floater in order to untangle the puzzle.
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King Nobby wrote:
More words please