When we eat food, it divides itself into three parts. The grossest part of it becomes excreta; that which is less gross becomes our flesh; and the finest part becomes our mind.
When we drink water, it becomes divided in three parts. The grossest part of it becomes urine; that which is less gross becomes blood; and the finest part becomes prāṇa, the vital force.
When we eat fire (i.e., butter, oil, etc.), it divides itself into three parts. The grossest part of it becomes bone; that which is less gross becomes marrow; and the subtlest part becomes speech.
‘O Somya, the mind is nourished by food, prāṇa by water, and speech by fire.’ Śvetaketu then said - ‘Sir, will you please explain this to me again?’ ‘Yes, Somya, I will explain again,’ replied his father.