Here are some possibilities:
* By volume: The second most common liquid on Earth is seawater. It covers the vast majority of our planet's surface.
* By mass: The answer is likely still seawater.
* By number of molecules: This is a bit harder to say, as we don't have an easy way to measure all the molecules in different liquids. However, a strong contender might be water, which is the most abundant liquid in living things.
It's important to clarify what "common" means in this context to get a definitive answer.
