You will be taught later that once an animal has been designated as a sacrifice, it is forbidden to substitute another animal in its stead, and if someone nonetheless does try to substitute another animal for it, declaring that this second animal is to be sacrificed in place of the first one, the first one is still offered up and the second one acquires the status of a consecrated animal, i.e., it may no longer be used for mundane purposes. What is then done with the second animal depends on what kind of sacrifice it was attempted to substitute it for. If the original animal was designated as a guilt-offering, then the second animal is not offered up, because it, i.e., the original animal, retains its status of superior holiness and does not share this status with the second animal. However, since the second animal cannot revert to its mundane status, it must be allowed to graze until it becomes blemished; it is then sold, and other animals are purchased with the proceeds from its sale. In this way, the sacred status of the second animal is transferred to the purchased animals, which are then offered up as communal voluntary ascent-offerings when there are insufficient obligatory sacrifices to keep the Altar occupied (which can easily happen on long summer days), since it is considered somewhat disrespectful to allow the Altar to be left inactive.
