C'est une règle de grand-mère

Grandma rules!

It took me a long time time to recover after the binge-drawing of Inktober .

Didn't really know what to draw, so here is a little episode from our French class. I was late as usual, and as I was entering, I heard our teacher say "C'est une règle de grand-mère", It's a grandmother rule. I wasn't really surprised, since surely some such rule is bound to exist.
Only a few minutes later, when we were told to look the rule up in our books, I realized it was not grand-mère, but grammaire that was ruling. 

To my ear these two sound remarkably similar.