When light passes from air to glass, some of it is refracted into the glass, and some of it is reflected. The reflected light is partially polarised because light polarised horizontally is reflected more strongly than light polarised vertically. In fact, at one particular angle of incidence called the Brewster’s angle, light polarised horizontally is 100% refracted, resulting in a 100% vertically polarised reflected light.
This phenomenon occurs for any transparent material, including glass, paint and water.
Further reading: http://en.wikipedia.org/wiki/Brewster%27s_angle