Things in the Game: Lava
Lava is very hot, so if you fall in, you're fried.
Fortunately, so are any other creatures that fall in, so if you can manipulate them into a lethal misstep, great!
If you must cross lava to get to a diamond, roll a rock into it. They don't sink and you will be able to walk across. But don't use a box, wood burns almost as quick as you.