Little tube which pokes out of the aeroplane to determine how fast you're going. On the ground you put a little condom on it to stop waspies and so forth from entering the system. If you forget to remove said condom, you can't tell how fast you're going and soon enough, soankeroo will occur.
What do you mean you can't tell how fast you are going? Just look out the feckin window and see how fast the clouds and birds are zipping by. Do you Airplane Johnnies ever actually fly sober?