"Bulletproof" glass is very different to ordinary glass. More correctly called bullet-resistant glass (because no glass is totally bulletproof), it's made from layers of polycarbonate (a tough type of plastic ), sandwiched in between pieces of toughened glass. This sandwich of layers is called a laminate . It can be up to ten times thicker than a single pane of ordinar...y glass and it's usually very heavy.
When a bullet strikes bulletproof glass, its energy spreads out sideways through the layers. Because the energy is divided between a number of different pieces of glass and plastic, and spread over a large area, it is quickly absorbed. The bullet slows down so much that it no longer has enough energy to pierce through—or to domuch damage if it does so. Although the glass panes do break, the plastic layers stop them flying apart. Think of bulletproof glass as "energy-absorbing" glass and you'll have a good idea how it works.