real analysis - Prove that $sum {{a_n}} $ converges iff the sequence of partial sums is bounded where $a_ngeq 0$

Let (${a_n}$) be a sequence of nonnegative real numbers. Prove that $\sum {{a_n}} $ converges iff the sequence of partial sums is bounded.

Uh I don't know how to do this proof. Please help!