Where do mansplainers get their water?
From a well, actually.
You see, this is funny because a "Mansplainer" (a portmanteau of "Man" and "Explain") very often begins their explanations with the words, "Well, actually".
The joke would have been a lot more humorous if I had added an ellipsis to the end. An ellipsis is three dots a the end of a sentence that indicates more would be said, but it is not relevant to the joke. I could continue...