What Christmas decoration, originally from Germany, uses heat from candles to spin its fan blades?

Christmas pyramid

Who portrayed Batman in the 'Dark Knight' trilogy directed by Christopher Nolan? What desert lies primarily within Mongolia and China? Which tuberous vegetable is thinly sliced and baked or fried to make potato chips? What day is Independence Day celebrated annually in the United States? What is the softest mineral known, often used in cosmetic powder?