There are lots of alphabet books out there. Matching a letter to an object and pairing them with a little bit of cute poetry is a conceit that goes back to the days when alphabet books were printed on a single sheet of paper protected by a thin layer of animal horn.

What makes the iPad/iPhone app X is for X-Ray different is its ability to feed kids' curiosity. Every alphabetic object in X is for X-Ray, from an accordion to a zipper, has had its insides photographed by Hugh Turvey, Artist in Residence at the British Institute of Radiology. (Which sounds like an incredibly cool job, to begin with.)

As you read through the book, you can turn the X-ray vision on and off, rotate some of the images 360 degrees around, zoom in on other images, and even put on a pair of stereoscopic glasses to see things in 3D.

Unsurprisingly, this gimmick works better for some letters than others. A flower, for instance, doesn't make for the most exciting x-ray to look at. Nor does a piggy bank. But the internal combustion engine more than makes up for those brushes with mediocrity. If you put the engine photo in x-ray mode and rotate it, the image comes to life. Suddenly, you're not just looking at the insides of a piece of mechanical technology, you're watching them work—pistons pumping and cranks turning. It's really neat and strikingly beautiful.

My main complaint with the app is really a complaint with app development, in general: X is for X-Ray is only available for iPhone and iPad. I don't own either of those things. If it weren't for the fact that Mike Levad, the app's producer, lives in Minneapolis and brought the app to me to try out on his iPad, I wouldn't even be writing this review. There seems to be a remarkable number of very cool science-related apps that aren't available for Android. I find that a bit annoying.

Watch the video to see a preview of X is for X-Ray.

Download it at the Apple App Store: $7.99 (iPad) and $2.99 (iPhone, iPod touch)