How to make something invisible

The mineral oil or vegetable oil that the presenter mentions here are a little less messy to use in the demonstration, but the index of refraction with those isn't quite as perfect a match as glycerine (or glycerol) is to the borosilicate glass.

In the summer camps we demonstrate this with three types of glass - fused silica, borosilicate, and soda-lime glass. It's only the borosilicate that disappears, so the type of glass you use certainly does matter.