If you went to middle school in the late 90s or early 2000s, you’re probably familiar with lenticular lenses. Remember those hologram stickers, posters, and trippy 3D trapper keepers? They were covered in ridges, and those ridges were an array of long, convex lenses, all running parallel to one another. The dimensional images in those stickers and posters are actually multiple images divided into thin slices, which are interlaced together. These magnifying lenticular lenses allow you to view those slices from different angles. From the left and the right, you can see the complete picture of each image, but as you turn the sticker the two (or more) images blur together, creating illusions—that’s how those stickers can make it look like a child is morphing into a monster, or a horse is running through a river.