Infants who are exclusively breastfed need no additional food or water. Contemporary medical professionals suggest adding other foods beginning between the fourth and sixth months of life, although supplemental foods in some cultures are offered much later during the first year. Unrestricted breastfeeding is an intimate experience involving extensive skin to-skin contact during which each participant sees, touches, smells, hears, and senses the movements of the other. Because of the frequent interaction, mothers and babies can develop an acute awareness of each other, sensing each other's moods and anticipating movements. For women, the complex hormonal, neurological, and emotional interactions associated with breastfeeding contribute to changes in blood flow and temperature, contractions of the uterus and other muscles, nipple erection, a sense of pleasure and well-being, and perhaps other responses not currently documented. Although it is seldom recognized, infants also respond physiologically and sensually. Human milk is quite diluted, and babies must nurse often to obtain adequate nourishment. Although it has been postulated that the dilution is an adaptation to ensure constant monitoring and stimulation to enhance the infant's cognitive development and a close mother-infant bond, dilution and frequency of contact are more likely the result of an evolving process combining physiological and behavioral factors.