One of the prohibited labors on Shabbat is borer, the separation of waste materials from items that one wants. Even during a meal, one must be careful not to pick out waste or unusable materials from food. For example, if there is meat on a bone, it is prohibited to remove the bone from the meat. Instead, the diner has two options: he may remove the meat from the bones, or he may put a piece of meat into his mouth with the bones and then remove the bones from his mouth.

In other words: On Shabbat, it is permitted to separate food from waste, but not waste from food. It is therefore permitted to pick the meat from the bones, but not the bones from the meat. The same rules apply when eating fish and other foods that contain inedible components.

The prohibition of borer applies not only to actual waste, but also to subjective waste, such as in a situation where there are two types of food and one wishes to eat only one type and does not want the other. For example, if one has a vegetable salad with onion and he does not want the onion, the onion is considered waste as far as he is concerned, and he is not permitted to remove it from the salad. He may still enjoy the salad by selecting the vegetables he wants from the salad and leaving the onions on the plate.

If an insect or dirt falls into a beverage, some authorities permit one to spoon it out with a little of the drink, while others prohibit it to be removed in this way. According to this stricter opinion, one must pour out some of the drink from the cup until the insect or dirt comes out.

When eating a fruit that contains seeds or pits, such as watermelon, it is prohibited to remove the seeds from the fruit on Shabbat, as this constitutes a violation of the labor of selecting. Instead, one should insert a slice of watermelon into his mouth together with the seeds, and remove the seeds from his mouth in the course of eating. One who is uncomfortable with this method may shake the watermelon over a plate so that all or most of the seeds fall off, and remove the few remaining seeds by hand. It should be noted that the allowance to shake a piece of watermelon and remove its seeds applies only when this is done right before eating the watermelon, not at an earlier stage.

If one eats fruit with a pit that cannot be shaken off from the fruit, e.g., a peach, and he wants to remove it before eating the fruit, he may remove the pit together with a little bit of the fruit itself.

Further reading: For information on the concept of selecting, see A Concise Guide to Mahshava, p. 170.