All that glitters...

All that glitters has a high refractive index

This joke is the deconstruction of the colloquial phrase "All that glitters is not gold". The original phrase is used to mean "just because something looks nice does not mean that it really is nice."

In this joke, the end of the phrase has been amended to be (perhaps) more correct. Things that have a high refractive index do tend to glitter, however gold (which definitely does glitter when it is polished) has a very low refractive index.