They process the world by what they can see visually.
Kids this age find it easiest to understand things they can see with their own eyes. They don’t understand that ice is frozen water. They don’t understand that a lump of clay can be rolled into a long rope and still contain the same amount of clay.
What you should do: Accept that you can’t explain everything to your child. Be patient, and don't lose your cool over the constant “Why?” questions.