Populate textbox from dropdown control selection asp.net visual studio 2008

I have a dropdown control with a number of options in asp.net page

If I select an option from the dropdown list I want to the selected text of the dropdown to be appended to a multi line textbox on the same form.

If I select another option from the dropdown control I want that selected text to also be appended to the textbox.  So the end result would be a multi line text box populated with say upto 6 lines.

I need a quick over view demo not a link to another site

Thanks