* Change in state, not composition: When water boils, it changes from a liquid to a gas (water vapor). The chemical makeup of the water (H₂O) remains the same. The molecules are just spaced farther apart and moving more rapidly in the gaseous state.

* Reversible: You can reverse the process by cooling the water vapor, causing it to condense back into liquid water. This reversibility is a characteristic of physical changes.

* No new substances formed: Boiling water doesn't create any new substances. It's just a change in the way the water molecules are arranged.

In contrast, a chemical change would involve the formation of new substances with different chemical properties. For example, burning wood is a chemical change because it produces ash, carbon dioxide, and other substances that are different from the original wood.