I generally keep an inexpensive (sub $50) pocket knife as the majority of the uses it receives, is opening packages. The other day my leg was bothering me, and I figured either my keys were rubbing me wrong, or my pen had come apart and was poking a hole in the pocket. Nope, I reached into my pocket and found my knife had come opened in it as my pinky slide across hit.
This made me wonder if there is a knife that locks shut, as well as open?