Bulletproof 'glass' is a see-through material that will stop bullets. There is a second kind of bulletproof 'glass' that only stops bullets travelling in one direction - bullets travelling in the other direction pass through more or less unaffected.

Naturally this is massively useful to people who worry about being shot at and might like to shoot back while remaining safe; security guards, bank tellers, the Pope, etc.

It works like this:

Take two sheets of clear material, one of which is extremely brittle and the other flexible. Laminate them together to form a clear, rigid sheet. This is your one-way bulletproof window. Shoot a bullet into it from the side with the brittle material first, and the brittle side shatters around the point of impact, absorbing some of the energy and spreading it over a larger area. The other material flexes and absorbs enough of the energy that the bullet is stopped.

Now step around to the other side of the window and fire another bullet. It hits the flexible material first and all the force is concentrated on a very small area, so the material flexes. On the other side is the brittle material which shatters outwards, away from the point of impact, allowing the bullet to burst through the flexible material and carry on towards the bad guy.

See, it's easy! And it's even been patented.