I'm currently student teaching and have been using a lot of lessons from CoP, so it's probably high time I shared some of my materials too! Here is a lesson plan I recently used with Jr. High students. This could probably be modified and used with any age level. It involves using 3 water bottles, one bottle with soil only, one water bottle with some plant material in it, and one water bottle with grass or another plant actually growing in the soil. Watch what happens when you pour water through each of the bottles!