I've always felt that a person who carries a knife openly is usually much less dangerous than the person who hides it. But I think Dan's comments are spot on. Part of the responsibility of carrying a knife is about social interaction. Like it or not, our socio-political environments often make carrying a knife somewhat difficult, especially when traveling.

If you are courteous, not flashy, and generally don't look like a grizzled crazy, you're doing everyone who carries a knife a favor by making it appear more socially acceptable. Any time I use one in public, I'm careful to take in my surroundings and keep it calm and cool. I hope everyone else does the same.

Now, even if I, a daily knife carrier (ok - 3-4 knives at any time) see someone at McDonald's pull out a 5" blade, I'm going to take notice and keep an eye on them. There are different knives for different places and it seems that it's sort of obvious to most, but perhaps not all.
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Experience is a hard teacher because she gives the test first, the lesson afterwards.