This popped through my feed today … quite funny – but it did make me think.
glassThe joke could have been even funnier (well I think so) by not changing the flow from ‘is’ to ‘contains’. Then we could have had;

“The Glass is 75% silica (SiO2), 13% sodium oxide (Na2O) and 10.5% calcium oxide (CaO). In add-on we could include 1.3% aluminum oxide (Al2O3), 0.3% potassium oxide (K2O), 0.2% sulfur trioxide (SO3), 0.2% magnesium oxide (MgO), 0.04% iron(III) oxide (Fe2O3), and 0.01% titanium oxide (TiO2).”

Or maybe that is just me being a stickler for consistency in joke flow.

Yahoo Answers :

Modern day glass is 75% silica (SiO2) which comes from quartz in sand, plus 13% sodium oxide (Na2O) and 10.5% calcium oxide (CaO).

Other minor additives include 1.3% aluminum oxide (Al2O3), 0.3% potassium oxide (K2O), 0.2% sulfur trioxide (SO3), 0.2% magnesium oxide (MgO), 0.04% iron(III) oxide (Fe2O3), and 0.01% titanium oxide (TiO2).