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When you first pour it into a glass(I reccomend this...) it will look like it has grit floating in it. Perfectly normal.

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Interesting feature of Guiness cans is that they have a plastic insert inside - sort of a bladder with nitrogen. When you open a can the pressure inside drops and the nitrogen is released from the bladder into the beer - very nice foam is created this way. So the foam on Guiness should appear regardless of the way you pour it into the glass.
I performed a surgery on a Guiness can and can confirm that indeed it does contain plastic insert :-)

Pawel