You post a head canon about your character, hogwarts, the magical world, anything. Then you name a subject topic thing or whatever and can include the word "other".. The next person writes a head canon based on that topic, or chooses "other" so that if you have a random head canon floating around your head, you can get it out and share it with teh world.
It doesn't have to be factual, it doesn't have to be realistic, it can be a joke or a parody, or play on stereotypes of the magical world, whatever. .
Headcanon: At Ilvermorny/Hogwarts, students don't "ask Alexa" or searrch Google for things. They ask Horned Serpents/Ravenclaws.
Summer: Zane's favourite things are summer storms, because he loves the thrill of flying through rain and wind, actually. Stupid daredevil, he almost was struck by lightening once because he was an idiot. His parents chained up his broom for a week after that and he felt like someone had cut off his legs.