"There is energy all around us. The energy is both yin and yang; positive energy and negative energy. Only a select few firebenders can separate these energies. This creates an imbalance. The energy wants to restore balance and in a moment the positive and negative energy come crashing back together. You provide release and guidance, creating lightning." - Iroh
DONE. One down, two to go. Bolin and Korra soon-ish.
Also, something tells me Mako is going to become a wicked-awesome lightning bender. I mean just look at him.