Hair

 When Brahmâ and the other Devas went to Visnu and prayed to Him then Hari gave to Brahmâ one hair of a black colour and one hair of a white colour. 

1. The Bhagavân S’rî Krisna was born of that black hair and 
2. S’ankarsana Baladeva was born of the white hair. They were both the incarnations of Visnu.

https://harekrishnainfo.blogspot.com/search/label/KrishnaBalarama

Source

https://www.sacred-texts.com/hin/db/bk04ch22.htm