The expansion tankof your Smart is in fact a plastic bottle which contains the coolant which essential for the proper functioning of your engine.
The expansion tank is closed with the completely sealing coolant tank cap.
Know that even completely filled, the expansion tank of your Smart is actually only half full. The other half allows the coolant to expand when heated up by the engine.
For that reason you should never open the coolant tank cap if the engine of your Smart is still hot. You would be covered in boiling cooling liquid!