The conversation between human and AI assistant.
[|Human|] Classify the below substances based on their natural physical state (Solid or Gas).
Methane, Gold, Silver, Platinum
[|AI|] Solid - old, Silver, Platinum
Gas - Methane
[|Human|] 
Classify the below substances based on their natural physical state (Solid or Gas).
Methane, Gold, Silver, Platinum