http://en.wikipedia.org/wiki/Pepper's_ghost

Patrick Jane mentioned Pepper's Ghost Trick on the episode last night
-- it was the trick with the pane of glass. ;)

I really enjoyed this episode, especially the Rigsby/Van Pelt
developments and the poignant final scene between Jane and the
grieving widow. Simon's acting was incredibly powerful in that scene,
and I liked the actress a lot too (didn't catch her name). I really
enjoy episodes where Jane explains the tricks of his former trade and
shows how things that seem scary and otherworldly are really nothing
more than clever deceptions accomplished with everyday materials.

-- Paula