Science

Corundum is Al2O3. Or maybe I just need more coffee this morning.

I wrote a stoichiometry problem for my chemistry students "the conundrum of corundum". Add a Cr impurity and a ruby forms. Add one of a number of different transition metals and a sapphire forms. The context rich problems always confused them - they were learning to separate important details from extraneous info (much like my post here).