I personally would prefer to rely on a home-made flint and steel than on a homemade neck lanyard. (ymmv :-) I presume the commercial ones for sale in Office Depot and similar places have been subjected to some sort of testing to guarantee that they actually will separate. A homemade one may not be testable. Of course, they're probably designed to break away at a fairly low tension; if you accidentally lose your office ID it's not a life-or-death mistake. But on the other side of the coin, having a breakaway lanyard that doesn't actually break away when you need it to could be rather serious.
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"The mind is not a vessel to be filled but a fire to be kindled."
-Plutarch