One object, three geometric shadows (Q69750)

I had once a dream where a wise man told me to stare at a wall which was illuminated by a spotlight. He put a little object in between, and it casted a perfectly round shadow on the wall. Then he spun the object and the shadow shape turned into a perfect square. Eventually he spun it for a second time and shockingly it casted a perfectly equilateral triangle. Could you guess how did the object look? (I have deleted the STL preview in order not to spoil you)