Add a text field to "Other" Checkbox

I have a Form with a Checkbox field where the user can select a few pre-defined options. I also have a catch all "Other" option in case the pre-defined ones fail to capture their interest. Right now, I the final option in the Checkbox field is called "Other" and I would like to add a text box where the user can input free form text. 

I know I can add a separate text box, but that doesn't tie in with the Checkbox field.

Any suggestions on how I can do that? 

Next level after solving inserting a text box would be to disable the text box unless the user selects the "Other" option.