Telling Twins Apart

I was listening to an item on the radio about the problems teachers have when teaching identical twins - primarily caused by them not being able to tell the twins apart.

I had a set of twins in my primary school class and teachers could do well to learn from my teacher, Mr Smith. He bellowed at one of the twins, who henceforth quivered ever so slightly when he was addressed by Mr Smith, thereby enabling him to tell them apart quite easily.