what happens when you light ashes on fire
Place a sugar cube on a fireproof surface (an ashtray, for example). Challenge your mark to set it on fire using a match. They won't be able to do it. When they give up in frustration, rub some fine ash onto the cube, then set it on fire. It will burn with a nice blue flame. Your mark may object, and say that it is the ash burning. You reply that it is indeed the sugar burning; the ash acts as a catalyst and is not burned.