Some day you may find yourself without the normal tools and devices that you might normally carry. In an emergency situation, when you have no knife, you can improvise a cutting tool by carefully breaking a glass bottle. As I have begun to learn flint knapping, I’ve begun to collect the bottoms of glass bottles as a modern material to work with. I’ll hold a bottle by the neck and strike the bottom rim of the bottle on the edge of a rock or concrete structure. This is done with more of a quick wrist action vs. an arm swing. This breaks off the bottom. Then I gently strike the side at a diagonal angle which leads to a sharp edged blade.
Any broken piece of glass has a sharp edge, but having an unbroken bottle neck to use as a handle is a real asset. Goggles and gloves are highly recommended for this process. Use extreme caution when performing this procedure and when using the resulting tool.