Have you ever sat down in an interpreter's booth, put on the headphones and tried to interpret the incoming speech? I did when I was a young and rather naive student who thought that being bilingual meant one could interpret simultaneously. No sooner had I started that problems arrived. As I was outputting the first sentence, the second one was already coming in but I hadn't paid enough attention to it. I remembered its beginning but not its ending. Very quickly I fell behind and I just couldn't say anything more after a few minutes!