CBSE Class 10 Answered

The most common cause of undigested food is the resence of fibres in food. While the body can break down most foods, fiber remains largely undigested. 

The digestion process begins with mastication. Therefore, if we do not chew our food thoroughly, it becomes difficult for digestive enzymes to work on the food, so they pass through intact.

Also, some foods has a tough exterior shell which makes it difficult to access the fats stored inside. As a result, these foods pass through our bodies whole.