The stomach absorbs extremely few chemicals, while it can take in modest amounts of some lipid-soluble molecules such as aspirin, other non-steroidal anti-inflammatory medications, and ethanol. The fat-soluble vitamins A, D, E, and K are also absorbed by the stomach. Water-soluble substances such as sugar, salt, and acids are completely broken down in the stomach into their component atoms and molecules, which are then absorbed through the intestinal wall. Some molecules, such as those containing a sulfhydryl group (--SH), will bind to proteins inside cells, causing them to become toxic. These binding interactions must then be reversed to allow for safe digestion and absorption of these toxins thereafter. Humans have four types of enzymes that are specifically designed to do this work: serine proteases, cysteine proteases, thioltransferases, and disulfide bond reductases. Of these, serine proteases are the most common and include elastase, trypsin, chymotrypsin, and cathepsins B, L, S, and Z. In conclusion, only certain foods are absorbed by the stomach while others remain intact for further digestion and absorption in other parts of the intestine.