What Turns a Sunflower's Head?

I have one very tall and handsome sunflower growing against my East fence. His face is wide and cheery and bright yellow. But to my dismay, he has decided to turn his head to face toward the neighbours below, instead of gazing into our yard of delightful flowers.


It got me to thinking the other day, what turns a sunflower's head? What causes them to gaze in a certain direction? Surely it is not the sun, since he is neither looking into or away from the sun's rays.

Next door, there is not much to look at:


Or is there? On closer look into the yard of the neighbour two houses down... There is this row of giggling, pretty sunflowers. Ahah!