Pressure element can be used to combine Black Hole, Boiler, Butter, Coal, Diamond, Eruption, Explosion, Granite, Juice, Paper, Stone, Volcano, and more in Little Alchemy 2. Making Pressure may be the simplest thing, you can easily get Pressure by combine Air and Air element. Although there are other ways to make Pressure, but I do not think they make sense. Check out the detailed walkthrough and try it yourself.
Air + Air = Pressure