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show your parents this picture, this is what a healthy pot bellied pig looks like, and then sit down with them and tell them you want them to hear you out, and take you seriously, a overweight pigs life will be dramatically reduced, he has very high chances of heart disease, and is most likely to have a heart attack, specially in hot weather. And as well with becoming blind, your pig can also loose its hearing, and when that happens your run the risk of having your pig become aggressive because it becomes afraid of its environment. and while you are having this talk, make sure you tell them how you feel about this, that you are scared for the health of your pig. If they try joking about it feel free to show your emotions and tell them they are upsetting you and you want them to take you more seriously.