Last night the Shikker Dovid went to a review of the issur of borar in the Talmud Shabbat and a review of practical halachot. The issur is removing bad or unwanted items from a mixture of things. Such an example is a bone out of pot of cholent. There is a story to give an example of avoiding choosing from mixtures. A rabbi takes a basket of fruit and tosses the fruit over a table to display them. One opinion is that he does this to make it look like a full table. Another opinion seems to be the normative idea that one should avoid choosing from a mixture.
Whats the problem with mixtures and pulling bad out on shabbat? The answer is in preparation. Shabbos requires preparation. A sign in my shiur room said, ” no holiness without preparation.” Pulling mixtures apart is proof that one did not prepare ahead, think ahead. Plan ahead. Of course, one cannot plan or do everything ahead of time. But by thinking of what food we serve, how we eat, how we choose things and prepare for them, we elevate our preparation and our holiness for shabbat.