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I chew my fingernails and keep them in my mouth (same as riffan)
And sometimes I bite off every nail and stick them between my teeth

I tend to hold either arm behind my back, as if I were a nobleman. I also stand on one leg, sorta like a flamingo

-When I drink pop from a can, I lightly open the tab then suck on it.
-I sit down in the shower, I know this is weird. I also sometimes fall asleep.

^
My brother does that 1st thing you said and I do the second

Every word/number/object/everything is associated with a color in my mind. They're obviously not all different colors, because that would be a LOT of colors.

But for instance; If you said car, I think yellow. House is blue, cup is white, hat is red, etc. With numbers it's different. I have colors for numbers 1-9 and then for double digit numbers the color is whatever color the first digit is. So 1 is yellow, 2 is blue, 3 is red, 4 is orange, 5 is green, 6 is blue, 7 is red, 8 is green, and 9 is red. So if the number were 42 then I think orange. If it were 78 I would think red. Idk why I do it, but that's the first thing I think of when someone says something.

Crack my knuckles, it's quite scary how often I do it. :/

I do exactly that, but with abstract images and diagrams that glow in my mind when I need to think about them. For example, the week is a disk composed of 5 parts in the top half which are the days of the week, and 2 parts in bottom half colored in red. Whenever someone asks me what day it is, the corresponding part will glow in my mind. That's why it's beyond easy for me to know what week day a certain day of any month corresponds to.

This happened a couple times. We were sitting in a group and someone asks the group what day of the week 25th of December would land on. Needless to say, it was around a month from the time he asked. Everyone got on their cellphones to see their calendars and answer the question, but I just made the different parts glow while counting the days til 25th of December, when I finish counting, I look which part is glowing and that would be the weekday that date lands on.

Ever since, all my family and friends ask me similar questions.

Long story short, diagrams and abstract images for almost everything are drawn in my head for easier counting/thinking.

Poking my nose, i love playing games naked, i also have tons of vanilla sugar in my tea, also mayo, cheese schnitzel on a fried sandwich with butter and ham.

Eating apples with core and all.
I think it's completely natural, like why would you throw away half of the apple and have a disgusting slimy thing you have to carry around until you find a trash can.
I also stuff all kinds of trash into my pockets. Like i'll go around with 3 empty cans of soda in my pockets.