The answer is water and cellulose.
Some parts of the food cannot be absorbed, so they left behind in the guts so they can later be excreted. Humans cannot digest cellulose because of the lack of some symbiotic bacteria that are able to digest it (such as cows have). Water is later resorbed in the large intestine, so it will pass excretory system (kidneys and associated tissues) before excreted. Therefore, cellulose and water will not be absorbed in the small intestine.