stored procedure not working

[Daniel Isaacs]

I'm trying to Pass Parameters with Procedures

the first query i run is

create procedure helloprocedure(OUT param1 VARCHAR(100))
begin
set param1 = 'Hello World';
end

then i run
call helloprocedure(@a);

and when i run
select @a;

i get
NULL instead of Hello World
is there any reason for that?