Multiple entry in one cell

I’d like to seek help from anyone to tell me how to insert multiple items (from a listbox) in, say, cell A1. Assume that I have a listbox that contains these items: “One”, “Two”, “Three”,… up until “Ten” (without the quotes). If I click the listbox for the first time and choose “Three” then A1 should contain “Three”, if I click the listbox for the 2nd time and choose “Six” then A1 should contain “Three” and “Six”, and so on.