Rails - Apr 2005 - n00b question regarding check_box

Just getting started with Rails by trying one of the tutorials
( todo list).  I deviated from the tutorial a bit and added this

  <table>
          <tr>
          <% for column in Todo.content_columns %>
              <th><%= column.human_name %></th>
              <% end %>
          </tr>
          <% for todo in @todos %>
          <tr>
                  <td><%= todo.description %></td>
                  <td><%= check_box("todo","done")
%></td>
          </tr>
          <% end %>
  </table>

to list.rhtml.  Problem is, no matter what the value of the
done field, the check box is not checked.  From the api doc
this looks ok.  I get the feeling I''m missing something really
obvious ( perhaps lack of coffee this morning ).  Any clue
for the cluelorn appreciated.

    ~Rob