Very astute. All living things are filled with water, and a powerful Waterbender can harness the water in plants or animals and control them. Things like glass, crystal and metal is all just various forms of earth.

As for lightning...well, some call it 'the cold-blooded fire'.

[Iroh closes his eyes and moves his arms around him in slow circles. The lights seem to dim from the room as electricity crackles between his hands. He exhales slowly, a long controlled breath, and the lightning dissipates into the air, leaving the hairs on Iroh's arm standing up.]

The elements are everywhere.