Fast food advertisements

I understand that the purpose of the pictures in fast food advertisements is to make the food look as good as it possibly can. This explains the presence of those perfect, six-storey-tall Big Macs (which actually are rarer than that three-titted woman in Total Recall).

What I don't get is why the soft drink in all these pictures always appears to be in motion -- to the point that it is spilling out of the glass. If we accept these advertisements as "best possible scenario" sort of things, why is spilling one's Coke a desirable outcome?