How do you explain Scientific things to a 1st grader??

Maybe what you want to do is put it into terms a first grader would understand: water is a liquid because it is runny (or another term), gas is just a liquid that a light or heat turns into water in the air, and solid is something that is hard; you can't push your finger through it. You can change any of those to suit your first grader/kindergardener if you need to.