Ever want additional questions to magically appear on your page based on how your user is completing your form? Many of our users have asked for this feature and we have added it. You can now choose to show/hide items on a page based on how items on that same page have been answered. For example, someone could check a box saying they will “provide additional detail”. Upon checking this box, a field appears to collect the additional information. You may want to use this feature to make a form look less intimidating by showing far fewer fields initially, As the user completes your form, more fields will appear.
See a sample here