A fear of balloons is very common in many with sensory sensitivities. Especially, as young children, but this can continue on into adulthood. Each new stimulus - object or setting - perceived as new, especially to young children on the spectrum, can cause strong reactions. My son was very balloon phobic as a child, but now will blow them up himself, loves them with helium in them, and even draws arty, funny things on them for birthdays, with a pen that could cause them to pop. I remember a brilliant autistic man who was equally terrified of planes. When taken to an airport at the age of 6, he screamed abuse at his uncle who was trying to drag him over to look at a light plane up close. Not much later, he became fascinated by planes, spending all his pocket money on kit planes. He was also terrified of the film Wizard of Oz(no surprises there ...
!) Usually the way to teach a young child to overcome fear of a novel situation or object, is to do so little by little, with gradual exposure. Just as you would teach a young horse to overcome fears, actually. If you force the issue, you increase the fear of the stimulus setting/object, and build distrust. After birthdays are over, I round up the balloons, place them in the laundry with the door shut behind me, and pop them swiftly, one by one, and get rid of them quickly ...
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Poor baloons.
I hate sudden bangs. Ooh. Working on bicycles. If a tyre explodes it leaves me petrified for days!