text_field_value

Hi,

I have a text_field_value that I want to set the value dynamically,
I use this script:
$(function() {
        $("#keypress").keypress(function() {
          alert("from keypress");
          $(this).value = "a" ;
          return false;
        })
})

the alert is shown but the value of the text_field is not changed
How can I have the right behaviour (for example every time I press a
key, I have the same key twice in the text_field)
regards,