This expectation is known as <span>A hypothesis
Hypothesis refers to the initial educated guess/explanation On why a certain event is happening.
This guess/explanation is still hasn't been backed through proper scientific testing and couldn't be held as objective truth.</span>
People are their most cooperative when interacting via this form of communication face-to-face.
<span>The concept of face to face communication is used in sociology, linguistics, media and communication studies.
</span>It is a technique of oral but informal communication. W<span>hen two or more persons talk to each other and see each other physically, this is face to face communication.</span>
It could be because the water is in constant motion, in someway filtering it with help from the sun heating the water, killing the bacteria.
If my High School memories come back to me correctly, then the answer should be the <em>American Civil War.</em>
Hope this helps.