As I've been doing commercial glass for the last 37 years, I can attest to the toughness of tempered glass. Tempering glass is similar to tempering and hardening metal. The glass is heated to high temps and the cooling and time is regulated. You can wack it with a hammer and it probably with not break, but hit it with something pointed and it will shatter into aquarium gravel, pieces still sharp by not life threatening if contacted. We used to throw salvaged door glass down a stone embankment about 30' high and usually they would make it to the bottom with shattering. I actually use a Starrett spring loaded center/marking punch to blow up tempered we put in our dumpster.