Not perhaps the most representative, and not from my favourite book even, but the scene in Crystal Soldier (Part 1, Chapter 3) where Jela shares his last water with the last tree. Depending on the time available and your preference, you can start as far before it as you want, and/or go to where he insists the tree comes with him when he's recovered.

Second choice, the bowli ball scene at the Academy where Theo figures out that their opponents are using a double-centred bowli ball.

Third choice -- To Cut an Edge