Knowing the three states of matter solid, liquid, and gas is an essential concept that all students need to know.
While this concept may seem intuitive to an adult, this idea is significant for learners given the relationship that the three states of matter have to one another and significant role that classification has in teaching. A fun introduction to teaching this topic is through the Wild E. Coyote science books. The specific book that deals with matter is Splat!: Wile E. Coyote Experiments with States of Matter (Wile E. Coyote, Physical Science Genius).