You show a Brass tube with a screw on cap. This tube is just large enough to fit a cigarette. At the center of the tube is a collar with a slot running through it.
You ask a volunteer to place a cigarette in the tube and ask him to run a blade through the slot on the collar. This cuts the cigarette in half.
You once again borrow another cigarette and repeat the routine of running a blade through the slot. The only thing you do differently is cast a spell on the cigarette. When the cigarette is retrieved from the tube, it comes out unharmed. Apparently you charm did work and completely restored the cigarette.
If you are doing this for a kid show, you could use a crayon instead of the cigarette.
This cigarette cutter is made of Brass on a precision lathe.