1) ''Erasing'' - ''Mochek'' - is one of the 39 Avot Melachot (Prototype Prohibited Activity). Erasing any mark (even a nondescript one) for a constructive purpose is always forbidden, regardless of what surface it is written on. Furthermore, it's forbidden to erase any legible script (or discernable figure), from any surface, even without a constructive purpose; a nondescript mark (like a blot of ink) may be erased only from a surface not commonly used for writing on, as long as the act has no constructive purpose. Included in ''Erasing'' are rubbing out, washing away, tearing or cutting apart and blotting out (for example, with ''white out''). 2) There is a dispute among the authorities as to what is the permitted method for cutting a cake with letters or pictures written on it (with icing, etc.). The practical Halacha is as follows: It's forbidden to cut directly through the letters or pictures on a cake, or to peel them off. However, it is permitted to cut between the letters, or to bite directly into a letter; it is also permitted to cut through decorations that are not meaningful shapes (like curved lines), or to cut into cakes which have been made into the shape of something (such as animal crackers). 3) And not destroy the lettering on Shabbat, because it has a practical function.

1) The letters are seen when the book is closed, opening or closing the book would break apart or form the letters printed on the edges of the pages, so violating the prohibition against ''Erasing'' or ''Writing''. 2) These authorities contend that since a book is made to be opened and closed constantly, doing so isn't a violation of the prohibition against ''Erasing'' and ''Writing''. However, if one has another copy of the book without letters printed on the surface formed by the edges of the pages, one should use that other copy on Shabbat (Mishna Berurah 340:17).

ספרים שעל חודי הגליונות מבחוץ נכתבו אותיות יש אוסרין לפתחם או לסגרם בשבת ויש מתירין וכן נוהגין ומכל מקום מאחר שיש אוסרין יש למנוע מלכתוב כן

1) Who sells produce. 2) The Sages prohibited a number of activities on Shabbat which usually involve the recording of details, so as to protect people from inadvertently violating the prohibition against writing. Included in this list of activities are long term loans, transferring ownership (e.g buying and selling, or giving a gift), and measuring and weighing. One may, on Shabbat, obtain an item from a store that one will need on Shabbat, as long the following conditions are met: a) There is no mention of any expression commonly associated with buying and selling, such as stating that one needs a product of a certain value or weight; b) There is no mention of payment. 3) The Rema 323:1 states that if the seller does not intend to make an exact measurement, it is permissible to measure the produce out in his own container, and then pour it into the cup of the purchaser. 4) Even if one gave money for the purchase to the Gentile before Shabbat, one isn't allowed to tell the Gentile (even before Shabbat), to make the purchase on Shabbat. If the money for the purchase was given to the Gentile before Shabbat, and one did not specify that the purchase must be made on Shabbat, and yet the Gentile chose to buy the item on Shabbat, there is a dispute among the authorities as to whether one is able to benefit from the item on that particular Shabbat (See Shulchan Aruch 307:4 and Mishna Berurah there. See also Ch. 73.