How do I create an open ended response after a multiple choice answer in my survey?

Hi
I have searched for the answer to this but I cannot find it unfortunately.
I would like to do the following in my survey:
Ask a question requiring a yes/no multiple choice answer, with a box underneath asking to explain their answer.
How do I do this please, I only seem to be able to choose one or the other, multiple choice or text box, and not both in the same question.
Thanks for your help.