The Grow series of games effortlessly taps into an often overlooked appeal of gaming: the idea of pressing a button or clicking on something just to see what happens. It's all deceptively simple: you click on an object to add it to the world and then watch how everything interacts. The joy comes in the way the relationships between objects are expressed: one object may transform another, which in turn transforms another, which affects the originally placed object in a cascade of delightful animations.
Grow Tower was released just two days ago, so give it a go. And if you've never played a Grow game before, check out the others as well.