In 1983, Fredbear's Family Diner opened. Chris Afton's birthday was happening there. That's when Chris' older brother, Michael says to him that Fredbear wants to give Chris a "big kiss." And so, Michael and his friends grabbed Chris and lifted him into Fredbear's mouth. That's when Fredbear's mouth closed and Chris lost his frontal lobe and died. That's the Bite of 83.

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So, apparently, some people think that either Mangle or Toy Bonnie did the Bite of 87. So, I'll explain how each animatronic did the Bite of 87 because I'm not sure who did this one. It's still debatable between the FNAF fandom.

𝗠𝗮𝗻𝗴𝗹𝗲

Mangle was tampered (or possessed to some others), therefore causing the erratic behavior she and the other toy animatronics have. The toy animatronics are normal towards kids, and since Jeremy was to be on day shift on Day 7 (After Night 6 and before Night 7), he would be bitten.

𝗧𝗼𝘆 𝗕𝗼𝗻𝗻𝗶𝗲

The Bite of 87 was an incident that occured in 1987 at the new Freddy Fazbear's Pizza, during the last shift of Jeremy Fitzgerald, when he was bitten by Toy Bonnie in the forehead, drastically damaging the frontal lobe.

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So, yeah. That's all.