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Brain in a Jar
  • Brain in a Jar item sprite
Stack digit 1.png
Use time20 (Very fast)
TooltipSummons a spider brain
'Pickled and shrunken, this brain can no longer hurt you.'
RarityRarity level: Fiery red
Research1 required
Grants Buff
BuffSpider BrainSpider BrainDesktop and Mobile versions
Buff tooltipIt's crawling around . . . icky
Summons Pet
  • Spider Brain
    Spider Brain
Dropped by Dropped by
Master mode icon.png Master
Brain of CthulhuBrain of Cthulhu.pngBrain of Cthulhu125%

The Brain in a Jar is a pet item that can be dropped by Brain of Cthulhu in Master Mode with a 25*1/4 (25%) chance. It summons a miniature version of the Brain of Cthulhu, the Spider Brain.


  • The Spider Brain slightly resembles the Brain Suckler, which latches onto player's head.
    • Likewise, this pet and the object used to summon it may be a reference to a scene from the movie "The Iron Giant," where the main character is watching a horror movie in which a scientist knocks over a brain in a jar. Upon returning to his lab, the brain is no longer on the floor - two seconds later, the brain, now having developed four legs, two eyes and a mouth, leaps onto the scientists face.
    • That being said, it is also possible, but less likely, that this item is intended as a reference to the arachnotron from the DOOM franchise.
  • The item itself may be a reference to the brain in a vat, a scenario in thought experiments which involves placing a disembodied brain in a vat of life-sustaining liquid and connecting it to a supercomputer, allowing the brain to perceive itself to be experiencing reality as if it were in the outside world.