Minecraft is a browser-based, first-person construction game, currently free-to-play in alpha. As it's so easily accessable and multiplayer-ready, it's about equivalent to an afternoon of messing about with Legos with one's peers: Incredibly Fun.
Here is an invisible kitten that I made.
Well, translucent, at least.
We spent at day making things out of cheese. These are just... samples.
Cheese skywalks (and other constructions) can cast interesting shadows. Grass only grows in the sun, and other materials interact in different ways.
Here's the inside of that cheese pyramid (above). There's a diving board directly above it that lets one fall through a helix onto this rainbow carpet. Trippy.
A giant cheese aqueduct leads from Cheesy Castle, pictured here...
...To a cheesoleum (See pic #3) containing Cheese Lincoln Memorial.