In the Hindu religion, all children when they reach the age of 9 or 11 must wear a sacred thread called a Janeiu (jnyaU). There were different types of threads, the higher the caste, the more ply thread was used. You wore a thread according to the caste you belonged to: 

Brahmins wore a 3-ply thread 
Khatriyas wore a 2-ply thread 
Vaishas wore a 1-ply thread. 
Shudras (lowest caste) were not allowed to wear a thread.   

      When Guru Nanak reached the age of 9, his father, Mehta Kalu (mihqw kwlU) went to see pundit Hardial Dass (hridAwl dws) to set a date for the ceremony of the sacred thread. The date was set, all relatives, friends and neighbours were invited over and a big feast was prepared.   

      On the day of the ceremony, there was a big crowd waiting. Pundit Hardial Dass came and set everything up for the ceremony. Guru Nanak was then brought out and made to sit on a little stool. Pundit Hardial performed the rituals and read scriptures while he prepared the thread. 
When the thread was ready, Pundit Hardial Dass tried to put it on Guru Nanak, but the Guru stopped him and asked: 

kOn AiDkqw Xwky pwey[iks pdvI ieh dyiq pucwey[ 
pihry ibnw nUnqw joaU[ idjbr muih smJwXo soaU[ 
                                                                        (Sri Gur Nanak Parkash) 
Translation: 
What do you get by putting this on? What new level do you achieve by wearing it? 
If you do not wear this thread, then how does it make you a lower person.   

Pundit Hardial answered, “This thread will show that you are from a high caste, it will get you respect from society.”   Guru Nanak then said, “I do not believe in castes. People are made higher or lower not by their castes, but by the deeds that they do in this life.”   The Pundit was surprised to hear the child’s answer, however, he still tried to convince Guru Nanak. He said, “Nanak, by wearing this thread, you get spiritual birth. If someone does not wear this thread, then they are not allowed to take part in any religious ceremonies.”   Guru Nanak then replied, “If the thread gives you spiritual birth, then it should not be put around the body, instead it should be put around your soul. What is the use of wearing this thread if you still keep on telling lies and doing sins. You need a thread that can clean your mind and make you a better person. This thread is made of cotton and with time it will get dirty and break. You need a thread that will never break, if you have a thread like this that I can wear around my soul, then I am ready to wear it.” Pundit Hardial got frustrated and said, “We are not starting this ceremony of the sacred thread for the first time. We have been performing this ceremony for a long time and no one has ever refused it before you.”   Guru Nanak knew that Pundit Hardial wanted him to wear the thread, because he would receive a lot of offerings of money and food for completing the ceremony. 
Guru Nanak stood up and looked at everyone and spoke the following hymns: 

            sloku mÚ 1 ] (471) 
dieAw kpwh sMqoKu sUqu jqu gMFI squ vtu ] 
eyhu jnyaU jIA kw heI q pwfy Gqu ] 
nw eyhu qutY nw mlu lgY nw eyhu jlY n jwie ] 
DMnu su mwxs nwnkw jo gil cly pwie ] 
caukiV muil AxwieAw bih caukY pwieAw ] 
isKw kMin cVweIAw guru bRwhmxu iQAw ] 
Ehu muAw Ehu JiV pieAw vyqgw gieAw ]1] 

Translation: 
  Make compassion the cotton, contentment the thread, modesty the knot and truth the twist. 
This is the sacred thread of the soul; if you have it, then go ahead and put it on me. 
It does not break, it cannot be soiled by filth (get dirty), and it cannot be burnt, or lost. Blessed are those people, O Nanak, who wear such a thread around their necks. You buy the thread for a few coins, and seated in your enclosure, you put it on. Whispering instructions into others' ears, the Brahmin becomes a guru. But man dies, and the sacred thread falls off, and the soul leaves without the thread.   

                      mÚ 1 ] 
lK corIAw lK jwrIAw lK kUVIAw lK gwil ]lK TgIAw pihnwmIAw rwiq idnsu jIA nwil ]qgu kpwhhu kqIAY bwm@xu vty Awie ]kuih bkrw irMin@ KwieAw sBu ko AwKY pwie ] hoie purwxw sutIAY BI iPir pweIAY horu ]nwnk qgu n quteI jy qig hovY joru ]2]   

Translation: 
  Even while wearing this thread, people commit thousands of robberies, thousands of acts of adultery, thousands of falsehoods and thousands of sins. 
They practice thousands of deceptions and secret deeds, night and day, against their fellow men. 
The thread is spun from cotton, and the Brahmin comes and twists it. 
A goat is killed, cooked and eaten, and everyone then says, "Put on the sacred thread." 
When it wears out, it is thrown away, and another one is put on. 
O Nanak, the thread would not break, if it had any real strength (power).  

Guru Nanak explained, what is the use of wearing this thread if you still steal and commit sins? This thread is only good if by wearing it you stop doing sins. Then he said that the true thread for our soul is the Name of God: 
  
                                 mÚ 1 ] 
nwie mMinAY piq aUpjY swlwhI scu sUqu ] 
drgh AMdir pweIAY qgu n qUtis pUq ]3] 

Translation: 
  Believing in the Name, honor is obtained. The Lord's Praise is the true sacred thread. When such a sacred thread is worn in the Court of the Lord; it shall never break.   

                       mÚ 1 ] 
qgu n ieMdRI qgu n nwrI ] Blky Quk pvY inq dwVI ] 
qgu n pYrI qgu n hQI ] qgu n ijhvw qgu n AKI ] 
vyqgw Awpy vqY ] vit Dwgy Avrw GqY ] 
lY BwiV kry vIAwhu ] kiF kwglu dsy rwhu ] 
suix vyKhu lokw eyhu ivfwxu ] min AMDw nwau sujwxu ]4] 

Translation: 
This sacred thread does not control lust, and no thread is given for women to wear. The man's beard is spat upon daily. 
This sacred thread does not control our feet and hands from committing sins, Nor does it clean our tongue from filthy words or control our corrupt eyes. The Brahmin himself goes to the world hereafter without a sacred thread. Twisting the threads, he puts them on others. 
He takes payment for performing marriages reading their horoscopes, he shows them the way. Hear, and see, O people, this wondrous thing. His name implies that he is wise, but he is really mentally blind. 

This thread doesn’t give us any wisdom, instead we are fooling ourselves by wearing this thread.   Pundit Hardial listened to these hymns and he realized that Guru Nanak was not going to change his mind, so he left without putting the thread on Guru Nanak.