As milk is made, the fat sticks to the sides of the milk-making cells while the watery portion of the milk moves down the ducts toward the nipple, where it mixes with any milk left there from the last feeding. The longer the amount of time between feedings, the more diluted that leftover milk becomes. This "watery" milk has a higher lactose content and less fat than the milk stored in the milk-making cells higher up in the breast.
As baby begins nursing, the first thing he receives is this lower-fat foremilk, which quenches his thirst. Baby's nursing triggers the mother's milk ejection reflex, which squeezes milk and the sticking fat cells from the milk-making cells into the ducts. This higher-fat hindmilk mixes with the high-lactose foremilk and baby receives the perfect food, with fat calories for growth and lactose for energy and brain development.