Auxin
Auxin is a chemical in plants that tells stems to grow tall and roots to grow down.
Think of it this way
Honey in a jar is heavy, so it sinks to the bottom. Auxin works the same way in a plant — it drifts down to the lower side of stems and roots. On the stem, this pulls the lower cells to grow longer, pushing the stem up toward the light. On the root, the same pooling slows growth so the root bends down into the soil.
Explaining auxin by grade level
Plants make a thing called auxin. It helps stems grow up toward the sun. It also helps roots grow down into the soil. Tilt a plant, and auxin bends the stem back up.