eyes in the back of my head
Often I'm asked "What's that (on the inside of my sunglasses)?"
It is the eyes in the back of my head, or more properly,
a rear view mirror.
On my biking glasses I have a similar mirror, being essential when riding. And as I've got used to it being there, I've also put one on the inside of my sunglasses.
On a pivot, they are stuck with double-sided tape onto the glass (plastic / whatever), and set-up. I super-glued the pivot so that it wouldn't change position when cleaning the glasses.
They can be purchased quite cheaply from Cambriabike.com