Enumerated types and sets of - counts and loops

:-)
Lets say I have this.

type
  TThing = (tgsOne, tgsTwo, tgsThree);
  TThings = set of TThing;

and then I do

var
  Things : TThings;
begin
  include(Things,tgsOne);

and that is all good, Things now contains tgsOne in the set.
then I do this
include(Things,tgsTwo);

now I want to know how many items are in the set.
showmessage( IntToStr(length(Things)) );
that dosn't want to work.

Also how can I loop the items in my set to see if they are in there/do stuff. Do I have to do
var
  Things : TThings;
  Thing : TThing;
begin
  include(Things,tgsOne);

  For Thing := low(TThing) to high(Thing) do
    If Thing in Things then
     //-- do something

:-)