A Shaking Car?

I was at the petrol station on the way home from work when I saw the car in front of me was shaking. I knew right away what was happening: the petrol station attendant was shaking the car so that he could put in as much as petrol as he could. Well, liquid does not behave like solid. If you were to pour sugar into a cup, you could shake the cup to cram in more sugar. You just can't do the same for liquid. All you get is a shaking car!