Polyunsaturated fat
fat or fatty acid having more than one double or triple bond within the carbon chain
Polyunsaturated fats are fats that are usually found in items like fatty fish, nuts, and vegetable oils. Such items are typically found to be liquid at room temperature.
Chemistry change
Polyunsaturated fats are long-chain organic compounds. Their molecules has more than one double bonds. The structure reduces the strength of the forces between molecules. This causes the items to have a lower melting point. This is why the items are usually liquid at room temperature.