This free app is incredibly engaging. You start with four elements (air, water, earth, and fire) and are required to combine elements to make new materials. Right now there are a total of 330 possibilities and that list keeps growing. While it is not a true scientific environment, it does afford the chance to think about the relationship between certain elements. For example, when you mix water and earth you get mud. Or when you combine lava with air you get stone. There are some silly combinations as well. My favorite so far is that a sword and the energy element combine to create a light saber. Add a human and you've got a Jedi!
The logic of this activity reminds me a lot of the crafting aspect in Minecraft. Some Middle School students at my school have been showing me how to create things in Minecraft worlds, and I soon learned about how intricate the crafting interface is.