Upnayan Sanskar is also known as Sacred Thread Ceremony. Upanayan is one of the traditional sanskars (rites of passage) that marked the acceptance of a student by a guru (teacher) and an individual's entrance to a school in Hinduism. The tradition is widely discussed in ancient Sanskrit texts of India and varies regionally. It is a ceremony in which a guru (teacher) accepts and draws a child towards knowledge and initiates the second birth that is of the young mind and spirit. This is also known as Janeu Sanskar. Nowdays, Upanayan Sanskar is been practiced with the Marriage ceremony. However, in Rishikesh there is a two day ceremony where priests from Varanasi perform the rites of Upnayan Sanskar. Upnayan is a ceremony where all the family memebers are involved and student receives a Sacred Thread known as Yagyopavita. With that a child enters the stage of formal education of Vedas in Gurukul.
Girls also goes Upnayan Sanskar in Hinduism. Girls who decided to become a student underwent the Upanayana rite of passage, at the age of 8, and thereafter were called Brahmavadini. They wore a thread or upper garment over their left shoulder. Those girls who chose not to go to a gurukul were called Sadyovadhu (literally, one who marries straight). However, the Sadyovadhu too underwent a step during the wedding rituals, where she would complete Upanayana, and thereafter wear her upper garment (saree) over her left shoulder.
The Sacred Thread (Yagyopavit) is a thin cord, composed of three cotton strands. It depends on the region you are living in as to how many strands are used. In some rituals, three strands are used to represent Brahma, Vishnu and Mahesh. When they are married, bride side gives three strands which represents three trinity of goddess - Pravati, Lakshmi and Saraswati.
There are several reasons why Upnayan Sanskar is justified. Religious and Spiritual reason - One feels steady that the god always with me in form of Janeu, I am the offspring of god. Janeu symbol of god existence with holder(who holds Janeu) which inspires a man to avoid misdeeds. Scientific reason - When a man goes for urinal, he hangs and folds Janeu over the right ear. There is a blood vessel named Lohitika on right ear, it is pressed by Janeu, which helps the urinal and excretion freely and reduces the diseases related to them.