Is your coolant level low? Refill the engine coolant correctly and safely
DO NOT open the cap on a hot car. The high pressure will shoot boiling antifreeze, causing severe burns. Instead, let the car cool and first check the cooling system pressure
Position yourself to the side of the vehicle, next to the fender. If the coolant does spray up, it will bounce off the hood and be directed forward. Use a rag/glove and open the cap carefully.
Refill your coolant with the correct type. Do not mix different colors of coolant. Newer VW/Audi use the pink G12 coolant. This is phosphate free / reduces production of hard deposits which clog the system and promote corrosion. Check the label, most coolants come as concentrates and should be mixed with distilled water.