I came up with a pretty creative idea:
An alien life form having an organ near to the brain that translates information out of the blood stream into feelings.
- The medium of communication: What molecules or salts do not react with hemoglobin and other molecules in the blood?
- Distinction of this medium: Somehow the organ has to distinguish the sent molecules out of the whole body to accurately tell where e. g. the pain is coming from.
Of course you can also describe how unlikely this evolution line must be.