Fuel is the burny stuff you see in a lot of OE-Cake videos. It burns. It's a solid material that can be combined with several other elements, and is often used to make Levels harder, by mixing it with the User element. As a solid, it floats, to a degree. To make a liquid out of it, you'll have to pour it, or mix it with an inflow of some kind. It's often used to make a sort of lava hazard, by burning Fuel from an inflow by passing it through Heater. This sets it on fire and it will then work just like Fire does.