​​​​Why does a Sunflower always face the Sun?

Facing of sunflower towards the sun is known as Phototropic movement. This is due to presence of auxin, a growth hormone in the stem. When the stem tip receives unilateral light, the concentration of auxin increases in the shaded side.
This causes the stem to become bulkier in the shaded region, so the flower ends up bending in the opposite direction – towards the Sun!