Next, put the water in a pot that all of the mussels and clams can fit in. Bring it to a boil and then add the shellfish. Clamp on the lid, reduce the heat to low, and let the clams and mussels steam for until they just start to open—just about 5 – 6 minutes, depending on their size, until they’re about halfway open. Remove the clams and mussels from the pan (or strain them out, reserving the liquid, if it’s easier)