Why does poop get harder the longer it stays inside the body?

Loss of water: Colon absorbs water from stool and it if sits in the colon it will get dehydrated and hard. Wish you good health! - Have a diet rich in fresh vegetables, fruits, whole grains, milk and milk products, nuts, beans, legumes, lentils and small amounts of lean meats. Avoid saturated fats. Drink enough water daily, so that your urine is mostly colorless. Exercise at least 150 minutes/week and increase the intensity of exercise gradually. Do not use tobacco, alcohol, weed or street drugs in any form. Practice safe sex, if you have sex.

That's it's job: The primary roles of the large intestines is water extraction & storage. If you don't empty often enough your stool will always be dryer & pebbly. Foods like apples, bananas & breads tend to produce slow emptying and dry stools. Some foods with more fiber trap more water & loosen stool.