Animal inside a house and ראובן stands by the door but does not fill it, and then Shimon comes and completes the blocking, Shimon is liable. If Reuven fully blocked the opening and Shimon also sat there adding security, only Reuven is liable. Even if Reuven leaves and Shimon is automatically left there blocking the door, Shimon remains exempt. This is no different from someone locking a door in his home when an animal is inside. Not only is Shimon exempt, it is permitted for him to sit with Reuven and continue to sit after Reuven leaves.

Usually, the term פטור implies it is still forbidden rabbinically, but there are 3 exceptions. a. Our case of Shimon blocking a door already blocked by Reuven b. Someone who makes hole in a boil- If the intent is to have a two- way opening for pus to exit and air to enter he is liable. But if the only intent is for pus to exit, he’s פטור. (אינה צריכה לגופה) It’s also permitted bc the rabbis were lenient for pain. c. Trapping snake(not life threatening) on Shabbos to use the snake after is liable. To prevent biting is פטור. Here as well it is permitted bc the rabbis were lenient with pain.

Most sheratzim do not give tumah when they die. There are 8 sheratzim mentioned in Torah that do have tumah of neveila. There are 3 opinions in Mishna in Chulin about the hides of these 8 sheratzim. a. R Yochanan ben Nuri- the hides of these sheratzim (unlike most sheratzim) are thick. Only the meat of these creatures is tamei and not the hides. Only the meat of a neveila is tamei and the hide is not considered meat. b. Rabanan- All 8 have thick hides but there is a גזירת הכתוב about the second pasuk of 4 that says their hides also give off tumah like the meat. c- R Yehuda holds we examine the physical makeup of the 8 sheratzim. 3 of the 8 have thin skins that is considered like meat and gives tumah and 5 of the 8 have thick skin that doesn’t give tumah.

This dispute is relevant to laws or Shabbos. Making a bruise is a melocho bc blood is “removed.” Even if no blood exits from the skin, we assume the blood has been removed but is just trapped under the skin. We only say this by creatures with thick skin that could trap blood, but by thin skinned creatures one isn’t liable for a bruise, bc we assume the bruise isn’t blood trapped under the skin. Regarding the 8 sheratzim, both r Yochanan Ben Nuri and Rabanan say all 8 have thick skin so making a bruise in all 8 is liable ok Shabbos, but acc to r Yehuda only 5 of the 8 have thick skin.

A wound on Shabbos is only when it is irreversible. But a bruise on thin skin or no bruise at all is not a melocho.

There’s a dispute about killing lice on Shabbos. The Gemara says they don’t reproduce, and the rabanan hold one is liable only for killing life that reproduces, but r Eliezer one is liable for killing lice.

Someone who takes fish out of water is liable for killing as soon as dry spot size of sela developed by the fins.

Someone who extends hand into animals womb and dislocated the embryo is liable. This is compared to detached hops from shrubs. One is liable for removing it from its natural area of growth even though it’s not detaching from the ground necessarily.