Tell them that glass splits up the different colors of white light and when different colored light passes through glass, it bends more or less dependant on its color.
Now write the words CARBON DIOXIDE on a piece of paper in capitals, CARBON in red and DIOXIDE in blue. Put the paper close behind the stem of a wine glass, and look at the words through the stem. The red letters turn upside down, but the blue ones don't. Your friends will be amazed.
Here's how it works:
As I'm sure you guessed this has nothing to do with the different colors of the words, or of red light being bent more or less than blue light. Actually the stem of the wine glass turns both words upside down, but because DIOXIDE is symmetrical about a horizontal line, you don't notice that it is upside down - it looks the same both ways.