Classify the below substances based on their natural physical state.
Water, Mercury, Copper, Oxygen, Nitrogen, Wood
Solid - Copper, Wood
Liquid - Water, Mercury
Gas - Oxygen, Nitrogen
Instruction: Classify the below substances based on their natural physical state.
Water, Mercury, Copper, Oxygen, Nitrogen, Wood
Context: 
Response: Solid - Copper, Wood
Liquid - Water, Mercury
Gas - Oxygen, Nitrogen