Let X, Y be normed spaces. Prove that a map T: X ---> Y is open if and only if T(B(0,1)) contains a ball around 0 \in Y

I think I can prove --> direction.
But I am struggling to show the map is open. I take an open set U in X. I have to show T(U) is open. How would I proceed from here?

Let X, Y be normed spaces. Prove that a map T: X ---> Y is open if and only if T(B(0,1)) contains a ball around 0 \in Y

I think I can prove --> direction.
But I am struggling to show the map is open. I take an open set U in X. I have to show T(U) is open. How would I proceed from here?