On one occasion when the children were little I was doing the washing up with my back to them, whilst they were eating their dinner and chatting.  Eloise and Kirsty suddenly clocked that I could tell who was speaking without looking, and for some reason they found this quite amazing.  In fact they had very different voices and it was not at all difficult to tell them apart.  They started to try to impersonate each other and disguise their voices to see if they could fool me.  We all had a good laugh as they tried out lots of silly voices, but try as they might they could not trick me!