The colon absorbs water from the solid waste that is left over after digestion.  Once water is absorbed, the remaining waste is expelled from the body.  If the smoothe movement of the waste out of the colon becomes disrupted, the result is IBS or Irritable Bowel Syndrome.  Sufferers of IBS often experience diarrhea and/or constipation.  Watch this video to get the full science behind IBS.