Threedoctorsareintheduckblindandabirdfliesoverhead.Thegeneralpractitionerlooksatitandsays,"Lookslikeaduck,flieslikeaduck...it\sprobablyaduck,"shootsatitbutmissesandthebirdfliesaway.Thenextbirdfliesoverhead,andthepathologistlooksatit,thenlooksthroughthepagesofabirdmanual,andsays,"Hmmmm...greenwings,yellowbill,quackingsound...mightbeaduck."Heraiseshisguntoshootit,butthebirdislonggone.Athirdbirdfliesover.Thesurgeonraiseshisgunandshootsalmostwithoutlooking,bringsthebirddown,andturnstothepathologistandsays,"Goseeifthatwasaduck."