I think the most impressive way I've seen are known as "ninja rocks" - broken pieces of spark plug ceramic insulator. But basically anything that can apply a fo

I think the most impressive way I've seen are known as "ninja rocks" - broken pieces of spark plug ceramic insulator. But basically anything that can apply a force to a tiny area of the glass can start a fracture that will propagate through the glass. They make little escape hammers and things if it's a concern.