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Answers: this is a very good question and I'm not sure but i think it's because homer is to dumb and is choking him wrong.
stupid answer but hey

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Because he's a cartoon.

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if you look close enough you see that homer is not actually choking him. If he was bart wouldn't scream like how he screams in the show

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Maybe Homer isn't squeezing hard enough or Bart has built up a tough neck from all the strangling over the years.

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He is used to it.

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They are just joking. If you watch The Simpsons Movie, when Bart accidently lost Ned Flanders' fishingrod he acts as his being choked... So Bart just fakes it that he's being choked...

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they said in the comentary for some of the seasons that at first the wanted to stay away from cartoon convetions like changing color when choking, they also didn't want homer to seem abusive so all chokings are impulsive and he'd either get out or choke him back, then later they didn't care so the chokings are longer but still no color change

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