DIOXIDE GLASS

Went to an open day for a prospective college for No.1 Daughter. The Physics area had some fun experiments including the "DIOXIDE GLASS" demonstration. Accompanied by some waffle about refraction of different colours, one is invited to think about the physics that invert GLASS but allow DIOXIDE to remain the right way up when seen through the "special" glass. In this case back at home, an old beer bottle full of water works as well.
As one of my rather splendid physics teachers used to say: "Physics is fun!"