the proof is in the pudding

Obviously we all found the old proverb ‘the proof of the pudding is in the eating’ way too long and roundabout a way to express the idea that you had to try something to see if it worked.  This expression is attributed to Cervantes in Don Quixote although not coined by him, and the Spanish phrase is ‘you will see it when you fry the eggs’.  It was translated into English and transformed along the way in the early 17th century.  The pudding at this point was not a Christmas pudding but a sausage, that is, ‘the stomach or entrails stuffed with a mixture of mince meat, suet, oatmeal, seasoning, etc., and boiled’. OED. The sausage might look all right but it was only when you tasted it that you could tell if it was off or not.

First the phrase was truncated so that you could just say the proof of the pudding and forget the eating bit.  The proof of the pudding for the new Metro is that people will start to use it.  But we were apparently not content with that and have adapted the short form to be the proof is in the pudding.  Now that we have forgotten about eating the pudding this makes little sense but we remember what the idea was that we wanted to convey and now this form of the phrase does the job.  It may even allow for greater flexibility in the sentence structure. You may imagine that you are a rock star but the proof is in the pudding.  This sentence would have been You may imagine that you are a rock star but the proof of the pudding lies in your future career.