Heartburn is cause by the improper function of the lower esophagus, contents of the stomach leak back into the esophagus." My question is how did you know what appears to be considered a fact?

Answer 1

actually, a fact a premise that can be proved, or disproved

In the above, "improper", "leak" are words that have no specific meaning..and cant be proved nor disproved. Finally, there is no valve, it is much more like a sphincter. Leak threw of the sphincter is expected.

Factual statement:
Heartburn results when acidic stomach contents enter the Esophagus, irritating the tissue.
Opinion statement: Stomach contents leak into the esophagus.
Fact Statement: Due to the fact that the esophagus lacks the mucus lining like that of the stomach, it can get irritated by stomach acid that passes the cardiac sphincter.
Opinion statement: Heartburn is caused by overeating.