A Candle on display is lit, for some part of another routine, or you can light it for this specific effect. It is covered with a small cap to extinguish the flame. When the cap is removed, the flaming Candle tip has vanished, and the Candle has transformed into a clear cylinder of Milk, which you can pour out. Or if you prefer, the Cylinder could be full of fine Salt or Sugar which pours out in an endless stream. Or it could even be a rolled up white silk ! Although not a very great mystery, this makes for a good quick visual effect with a surprise climax. You could also blend this into another more elaborate routine. Supplied all the required parts (You use your own milk or salt), with instructions at a very low price.
