In the mood for a little visual trickery? Click through to see four new optical illusions that'll challenge your perceptions and stimulate your thoughts.
SIDEWALK SPHERE: Created in Paris by Francois Abenalet, this ingenious sidewalk installation goes beyond the usual chalk drawings by incorporating actual grass and trees. It's a classic example of anamorphosis -- a distorted image that only takes its "proper" form when viewed from exactly the right angle.
GEORGE WASHINGTON: America's first president gets a little homage in this illusion, which includes both a picture and a signature that work equally well when viewed upside-down.
HIDDEN ANIMAL: Can you spot the creature embedded in this tree? If not, click here for a little help.
IMPOSSIBLE ROOM: LEGO mastermind Henry Lim has used the plastic blocks to create a convincing three-dimensional version of M.C. Escher's famous mind-boggling lithograph, "Relativity."