On is used when one thing covers another or lays on the top of it. Although, this definition matches the definition of On and On often replaces over when it's used to convey 'on top of.
Using "On"
"On" is used to indicate position, usually indicating that something is on top of something else. We might say, “My journal is on the desk.” In this function, “on” typically denotes proximity or position. Another example would be, “He sat on the stone wall.”
Over
Over for movement and position. We use over to talk about movement or position at a higher level than something else: ...
Over and all over. We can use over, and the stronger form all over, when something is covering something: ...
Over for periods of time. We can use over to refer to extended periods of time.
because tina was telling the class about her and her group fought in history class & sarah said to herself sounds boring which means that tima heard it and turned around angrily because of what sarah said & that was the main conflict
