Glass can be hard enough if it is break resistant if it can drive nails on the outside. However, if the force used us big enough, then it can easily break. If the force is not big enough, then it cannot break.
6 months ago
Last edited at 6:11AM on 9/20/2013
The fact is that glass is not a crystal, it is a liquid(or let's say rubber) in a very solid state. Which means the bonds between the substances vary from area to area, and of course depends on the temperature and the mixture of compounds that makes the glass. The fragility here depends on the way the material emits and transfers the sound waves inside. If a certain hit creates sound waves that come over at one point the intensity might reach a lot higher then driving a nail at that small point which might break the glass easily. If the waves gets distributed homogeneously on the other hand, the force at each point might be lower than necessary. So both can happen using the same piece of glass.