Drinking a Liquid while Sitting
The Prophet (pbuh) noted that we should not drink while standing. (Imam Muslim)
The stomach of a human has a different position while sitting and standing. If a human drinks a liquid while standing, the liquid directly passes to the duedenum through the pilor. However if the human drinks the liquid while sitting the liquid first remains some time in the stomach before passing to the duedenum.
The amount of harmful organisms in the liquid is decreased as the liquid remains in the stomach because "some" harmful organisms in the liquid are eliminated by the acidic environment of the stomach. Off course, the stomach cannot eliminate all harmful organisms but still drinking while sitting is healthier than drinking while standing.
The Prophet (pbuh) noted that we should not drink while standing. ( Hadith, Imam Muslim)
Drinking while sitting is desirable but not obligatory. The Prophet (pbuh) sometimes drank while standing to explain that doing so is also permissible.
( pbuh = peace be upon him )
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