Water

Water is a commonly spawning liquid in both minecraft and tekkit. It is very useful throughout the modpack in many different mods.

If you right click a block of water with a Empty Cell, the the cell turns into a Water Cell, and the block dissapears. If you put blocks of water around your Nuclear Reactor, it will be cooled! (The more water the more cooling, and only if it's adjacent.)

You probably know the old-fashioned way to make obsidian, but here's another way:

(Shapeless recipe)