ddddddddddddddddddddddddddddddddddddddddddddddddddd

With limited information, here is my academic guess,
let's assume, there is only one man in the world.
The money he puts into his bank account is principle.
At the end of each period, he will get interest which makes his total deposits bigger than before.

How does bank grant the interest?
It actually comes from bank's earning via utilizing the deposit. (borrow low lend high)

Is this what you asking for?