Gravitropism
Gravitropism is how plants sense gravity and grow their roots down and stems up.
Think of it this way
A sealed jar holds water with a small ball inside. When you tip the jar, the ball always floats to the top. Plants use gravity the same way — roots grow down toward the pull, and stems grow up away from it. The plant always knows which way is "down," just like the ball always finds "up."
Explaining gravitropism by grade level
Turn a plant on its side. Watch what happens. The roots bend down toward the ground. The stem bends up toward the sky. Plants can feel which way is down. They always send roots that way.