Answer:
Loan it out to someone else at a higher rate of interest than the rate you are earning
Step-by-step explanation:
The principal way commercial banks earn money in a fractional-reserve system, is by obtaining money from the public, and loaning out a fraction of that money at a higher interest rate.
A commerical bank may pay for a deposit an interest rate of, for example, 2 o 3%, but when this commercial bank loans out part of that deposit, it may charge 5, 6, 7% or even more.
This is why commercial banks tend to be very profitable businesses most of the time.