one way windows

How do you create one-way windows?

Place an alpha window texture on one side of a prim, and a non-alpha texture on the other. For example, I used to have a lighthouse that the second floor bedroom window, from the outside, always showed the curtains as closed (Non-alpha texture with curtains drawn). From the inside, I had a texture with no curtains (alpha texture with transparent window panes), and a set of prim curtains in front of the window that I could open or close. This allowed me to open the curtains and look out, but no one outside could look in.