We live in a three-dimensional world, but could there possibly be more than that, ones we can't perceive? Definitely. Want to try to perceive them? Then download The Fourth Dimension. This interactive app explains what how a fourth spatial dimension would interact with ours, and how we would see it in a three-dimensional context, by partially comparing it to how objects can appear three-dimensional in a two-dimensional plane (like any 3D video game, or just a drawing of a cube on a piece of paper), and expanding it out to a fourth-dimensional object's projection into three-dimesnions.
Or at least I think that's how it works. It's all very hard to grasp. Thankfully, the app authors tackle the subject with a goofy wit that should help keep users' brains from melting into goo. The ability to interact with shapes, and get a feel for how fourth-dimensional tesseracts would appear in a three-dimensional world lends the app a degree of helpfulness that a scientific book, or even a novella like Flatland cannot.