Does anyone know how to create forms that change based on form inputs? For example, certain parts of the form are hidden until a certain box is checked and then the previously hidden part of the form expands so that the user can fill it out. Does anyone know how to do this or what these types of forms these are called? I am basically trying to finish a checkout page where I would like a series of inputs for the user's "billing address" to appear if the user unchecks the box marked "Billing address same as shipping address."

Thanks for your help, Bill