The majority of living things are divided into two categories: plant or animal. As students witness the huge diversity–microscopic to huge–within each group, challenge them to identify the single feature that distinguishes plant from animal. It is their source of energy (what they require to make them “go”): if it gets its energy from feeding on other plant/animal material it is animal; if it obtains energy directly from sunlight (photosynthesis) it is plant. (Other kingdoms and exceptions to this should be left to later lessons so as not to overwhelm young kids with too much information.)
A common misconception is that plants get energy from feeding on soil. See the text to guide kids through observations and reasoning that make this idea false.