Tree of Knowledge and a very good question

We have been reading a children's Bible (the Jesus Storybook Bible) to Elliot who is 5. In fact, we finished reading it to him and he just wanted us to read it again. We usually read 2 or 3 chapters a night.

A few days ago I was reading the chapter titled "The terrible lie" based on Genesis 3. This is the story of the Tree of Knowledge.

Now, God had given Adam and Eve only one rule: "Don't eat the fruit on that tree," God told them. "Because if you do, you'll think you know everything. You'll stop trusting me. And then death and sadness and tears will come."[from the Jesus Storybook Bible]

Elliot knew that God had created everything, including the plants in the garden. The story goes on to describe what happened when God finds out that Adam and Eve disobeyed Him and ate that fruit.

Elliot asked, "Why did God put that tree in the garden?"

My answer? "That's a VERY GOOD question! I don't know!"