What sorcery is at work here? Careful, this trick -- er, illusion has a second, surprising twist in it. Silhouette Zoetrope is the work of Christine Veras at the Nanyang Technological University in Singapore. It was one of the top three illusions in this year's Best Illusion of the Year contest, in which scientists compete with each other to create the most confounding illusions.
At first, Veras' illusion appears to be a relatively simple one. You see one bird flying inside the cage, but the bird is actually several still birds outside of the cage. It's the rotation of the wheel combined with strategically placed slats of light that make it appear to be inside instead of out and moving instead of still. Easy enough, but watch all the way to the end, and you'll see a second illusion embedded in the first.
The bird you see flying is facing to the right, but when Veras stops the zoetrope and you see the birds, in various stages of flight, are all facing to the left. It's only when you see them in rotation through the slats, that they appear to be flipped.
This illusion is the third place winner of the Best Illusion of the Year contest, the results of which were just announced. You can see all 10 right here, including this marvellous little bit of trickery from Kokichi Sugihara from Meiji University (whose work we've covered before.)