This is classic: A Guide For The Un-Initated To Buying Guinness In An Irish Pub.
3 The barman will fill the glass between 70% and 80% capacity. It will then be put to the side for a few moments to allow it “to settle”. Once the brownish liquid has almost turned to a solid black the barman will then fill the rest of the glass. NB: do not under any circumstances take the glass before it is filled. Some virgins seem to think that the settling stage is the final stage and walk away with an unfinished pint. At this point we Irish DO understand the predicament, but I assure you it causes endless mirth as well.