The acidic, fluid mixture of gastric juice and partly digested food in the stomach is called chyme. A few substances can be absorbed across the stomach wall, including alcohol (hence its rapid effects), aspirin, and caffeine, but even these substances are absorbed in rather small quantities from the stomach; most absorption takes place in the small intestine. At the bottom of the stomach, peristaltic contractions push the chyme up against the pyloric sphincter. Associated waves of anticipatory relaxation cause the pyloric sphincter to relax briefly so that little squirts of chyme enter the small intestine. The human stomach empties gradually over a period of approximately 4 hours. This slow introduction of food into the small intestine enables it to work on a little material at a time.