Populate a single sheet with multiple entries from a drop down list

Hi this is my first posting on here so I am hoping someone is kind enough to help me. I am OK at putting a drop down validation list in my Excel 2007 spreadsheet but what I want to do is be able to chose multiple choices and these choices made populate a single cell followed by a comma each time a choice is made.
Sorry the title should read Single Cell not Single Sheet !! oops a senior moment