Our planet is always changing, right under our feet. But how do you make these big, abstract concepts meaningful to a curious young child? Here's the perfect app. The fifth in Tinybop’s "Explorer’s Library" series is also one of the best (we also highly recommend The Human Body). You start with a translucent model of the earth that spins with a swipe. You can cut it in half with a cross section tool, and there's a geological time line, so you can see how the Earth looked when it was first forming. As with the other Tinybop apps, your finger drives the concepts (not the other way around). The good stuff happens when you zoom in to a detailed depiction of our planet's surface, and touch one of 12 magnifying glasses or windows that zoom into a detailed scene. Many let you drive a specific process. Our favorite -- the volcano where you can change the shape and height in real time, and tap the lava to make it erupt. You can make ice form in crevices to make a rock fall, or launch an iceberg by making waves. Other nice touches include a vocabulary feature, with drag drop labels, and a detailed PDF users manual that helps you better understand each process. The distinctive illustrations are by Sarah Jacoby. Vocabulary is in more than 40 languages with interactive labels. The botton line? This is a great way to let children informally and playfully discover some of the big ideas they'll encounter later on in their school curriculum. It's a great use of $2.99.