The skeleton gives us our shape and keeps our body upright. Without the bony framework, we would probably have looked like a flabby jellyfish on land, because, after all, one-fifth of our body is water. We differentiate our skeleton into the skull, the body, and the limbs. The first two protect us against injuries from outside, and the limb skeleton primarily takes care of our movement. The bones of the skeleton are joined by ligaments. We move with the help of muscles. Muscles become stiff and ache after a vigorous workout.
Urine is a waste product, which is formed in the kidneys after the purification of blood. Kidneys contain millions of very tiny canals, called nephrons, which filter out harmful substances as well as excess water from the blood to form primary urine. This liquid still contains some useful components that are reabsorbed into the body in the kidneys. The remaining fluid that can no longer be used flows through the ureters into the bladder. Once the bladder is full, we need to go to the toilet and relieve ourselves.
Stress, an unhealthy way of life, bacteria, and viruses are the most important causes of intestinal problems. Bacteria and viruses reach the body via food or water and cause sickness, vomiting, and diarrhoea. Diarrhoea is caused because the body fights against these invaders and wants to get rid of them as quickly as possible. Sometimes, with diarrhoea, we have fever too, since an increase in the body temperature accelerates the healing processes of the body. 
After the absorption of the nutrients in the small intestine, the 1.5 m long large intestine absorbs water from the remaining undigested food and converts it into a solid mass. The solid mass of waste matter is stored in the lowermost part of the large intestine, the rectum, and is then excreted as ‘stools’ through the anus.
The function of the small intestine is to digest the food into simpler nutrients, and to absorb these nutrients. If food stays longer in the small intestine, it will have more time to get digested completely and will also enable the body to absorb all the nutrients. The very long length (more than 6 m) of the small intestine enables the food to stay there longer. The finger-like ‘intestinal villi’ on the inner wall of the small intestine help in absorbing the nutrients in the bloodstream. The liver and pancreas produce enzymes that travel to the small intestine to help break down proteins, fats, and carbohydrates into sugar. This process is known as metabolism.
After the food has been chewed and mixed with saliva, it reaches the stomach through the oesophagus. The food is prepared for actual digestion in the small intestine, which is connected to the large intestine, and ends in the rectum. The undigested remains of the food and urine are excreted from the body. We can survive without water only for a few days, but the body can manage without food for quite some time. If the starvation goes on for a very long time, the body becomes weak and prone to diseases.