A group from the National Institute of Advanced Industrial Science and Technology (AIST) has designed glass which can switch between window and mirror. The switch is done by altering the gas content between the two glass panes. By introducing a small amount of hydrogen into the atmosphere between the panes, the glass acts as a transparent window. Alternatively, adding a small amount of oxygen with no hydrogen forms a reflecting mirror. The idea is to save energy in buildings by reflecting away heat on heavily sunny days. But really I'd just use it to spy on women in my bathroom. Wait, I didn't just say that.