Before I educated myself on this subject, I literally use to assume that farm animals were (surely...?) just led into a room, and peacefully put to sleep via lethal injection.
I had no idea all of this was going on:
A "captive bolt gun" is used to induce unconsciousness in animals before they are killed.
Many animals however are not stunned properly and continue through the slaughter line in a conscious state.
Animals are then strung up to hang by their ankles, and hacked apart.