The breasts of a woman also play a role in human sexual behavior. They are one of the most visible or obvious female secondary sex characteristics. They play an important role in sexual attraction of partners. They can also give pleasure to the individual. When sexually stimulated, the size of the breasts increases, venous patterns across the breasts become more visible, and nipples grow harder. During sexual intercourse, it is common practice to press or massage breasts with hands. Breasts are sensitive to touch as they have many nerve endings. Oral stimulation of nipples and breasts is also common. Some women can achieve "breast orgasms". In the ancient Indian work the "Kama Sutra", marking breasts with nails and biting with teeth are explained as erotic.