Because the joke, you see, is... sorry, we can barely stop ourselves giggling... The joke is... wait, no, sorry... still laughing too much...
The joke is: the hand action when you're shaking up a For Goodness Shakes protein drink is a little bit like the hand action when a man is masturbating.
Well, don't worry if you don't get it - because the joke will be repeated over and over again in this advert (usually with the man acting like that near a woman).
You can watch it all unfold, as it were, above. And once you have, tell us what you think in our poll and in the comments below...
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