As a light source emits light waves directly toward you, the waves vibrate at every angle: horizontally, vertically, and all angles in between. By placing a polarizing screen in front of your eyes, you can block horizontally oriented waves while allowing vertically oriented waves to pass. This is how polarized sunglasses work to block reflected light waves, which usually vibrate horizontally but not vertically.
What is a good way to demonstrate how a pair of polarized sunglasses works?