Ripley: [On Ash's tracking devices] What's it key off?
Ash: ... Micro changes in air density.
Ripley: [Later, when trying to track the alien] 'Micro changes in air density', my ass.
Ash: ... Micro changes in air density.
Ripley: [Later, when trying to track the alien] 'Micro changes in air density', my ass.
Ripley : [On Ash's tracking devices] What's it key off?
Ash : ... Micro changes in air density.
Ripley : [Later, when trying to track the alien] 'Micro changes in air density', my ass.
http://www.moviequotedb.com/movies/alien/quote_26921.html