Learn how NOT to believe what you see with this book of eye-popping pop-up visual tricks.From the classic "which dot is bigger?" or "is this line straight?" illusions to the first-ever 3D trick to appear in a book, Optical Illusions is packed with picture puzzles. Every illusion is set up to challenge what you think you see. There are hints and tips of how to approach the illusion, and then an explanation of how it works and why you were fooled (and you will be!). There are also flaps, tabs, and a decoder to test the answers, so you can check whether different-looking colours are actually the same, or if those lines really are wonky. All kinds of visual tricks are featured, including distortion, motion, colour and brightness, ambiguous illusions, impossible objects, hidden images... but the stars of the show are two large interactive illusions - the "thaumatrope" spinner that puts a bird in a cage, and a 3D perspective sculpture. Even though you will build it yourself and know which bits pop out and which parts are flat, you can't help but see them the other way around.Optical Illusions will get you lifting flaps and turning tabs in your quest to see the truth behind the illusion - and the more you discover, the more impressive the illusion becomes.