They want to know “how” and "why.”
By third grade, children have been taught much of the “what.” What’s two plus two? What letters are in the alphabet? What’s the difference between fiction and nonfiction? It’s time for them to learn “how” and “why.” How does a plant grow? Why is multiplication useful? Why does the Earth spin?
What you should do: Be patient and answer questions whenever possible. If you don’t know the answer, look it up to show that adults are also curious and that learning is a nonstop process.