What’s a pixel?
How big is an em?
What about rem?
How big are they?
I sometimes have to explainn the differences between these, particularly when to use them. I made a little demo page to help
Try this http://is.gd/pxorem
There’s four bars that are direct children of the body, and they are repeated inside a div. You can set the font size of the body and the div, and set the widths of each in em, rem, px and %. It’s a bit clunky but it might help someone.