So, apparently when molten hot glass is dropped into a bucket of cold water, it acquires so strange physical properties. Its compression strength (i.e. getting pounded by a hammer) gets many times stronger than normal glass, but its sheer and tensile strength (i.e. breaking of its surface tension through cutting or stretching) gets many many times weaker. In fact the slightest nick can cause it to explode.
Smarter Every Day has the super hi-speed/high-res footage to prove it.