How to add a new field with an "if statement"

How to add a new field with an "if statement"

I would like to know if it is possible to have a new text field to appear on my form when a user selects a specific item from a drop down menu.

Example:

If the user selects "other" from a drop-down menu, I would like for a new single line field to appear labeled "other" so that the user can fill in the required information....

Please help.

Thank You in advanced