Box-cutter knives

A box cutter knife is a common tool used in grocery stores and other places. Such a knife generally consists of a simple and cheap holder which can hold a standard razor blade. When the razor blade becomes dull, it can be quickly switched for a new one.

A box cutter is typically flat, approximately 1 inch across and 3 or 4 inches long.

Such a knife could be easily concealed in carry-on luggage, made invisible by lying it flat next to any common metal object, such as a can of hair spray.