The most advantage i guess is you can avoid the most embarrassing situation which often  happens to me.Quite often , i have seen a friend or an acquaintance from a long distance.of course , i was being ready to wave my hand or do something else to greet him or her. And the result is they often did not see me ,i was likely to do a stupid thing. thanks to God , i have precise eyes. i know many of you who are short-sights may want to "punch"(i am not sure ,someone know please tell me . it means beat by your fist) me. but i get enough of this embarrass situation.

    oh , i am busy now`````````write it neXt time``