Water provides so many benefits to our systems. Depending on age, activity, and wellness, people are anywhere from between - 50% - 75% water.
When matter leaves the small intestines and enters into the large intestines, the matter is liquid. The large intestine's job is to remove water and eliminate the matter painlessly.
If you are already dehydrated, then the matter that has moved into the large intestine already has low water content. The large intestine removes every last drop of water and the matter becomes rigid, stony, unyielding causing you to be uncomfortable or in pain. If a person is unable to pass and eats more, then the issue is exasperated and becomes more compact and more unyielding and more painful and could become more critical.