In early times there was a bhagwati devotee named “Nandlal” lived at village narela near delhi (India). He spend his hole life in worshiping Maa Bhagwati. One day after his prayer he cut his head and offered to maa. Goddess Durga appeared. She said that sh is very impressed with the devotion (bhakti) of Nandlal. Maa Vaishno Devi asked his wish. Toay Godess wanted to gift him his wish.
For the welfare of people he replied that in future era no devotee will be able to do the same as his early devotees Bahagat Dhayanu, Dugdadhari Bhagat has done. Please find out someway so as in future no devotee of urs need to offer his head to you. Then Goddess bless him and give this fruit called "Shri Fal” (nariyal/coconut) to him and told from now the devotee who will offer this nariyal to her she will accept it as his Sheesh((head).
This fruit resembles the person head. The outer covering is similar to humans head hairs. After you remove tha you will find three spots which shows human Two eyes and mouth. The ruit Nariyal coconut is filled with water inside is just we have blood running in our body.
That is why it is said always use the nariyal which have water in it to offer to goddess Bhagwati.