Thanks for the laugh!
It is really funny reading about all these peculiar things in Germany from a German perspective.
About pillows: I actually prefer the German pillows, the American ones are too small, they cannot be squished and accustomed to the shape you want them to be.
About Kreislauf issues: do y'all never feel dizzy/ have low blood pressure? That's what it is. I rarely have that problem, but sometimes I do.
About train seats: I hate when the entire train is empty and someone decides to sit right next to you. Sometimes I then get up and sit down on another seat.
About ironing: I do not iron anything. That is also why I don't wear my nice shirts a lot, because they're too crumply to be worn without ironing. But bedsheets, underwear, socks, PJs and towels, hell no!
About air conditioning: I think that is a matter of what you are used to. When I moved to the US I got a cold three times within a month. Just when I was feeling better, there it was again. Most Germans are not used to being cold inside. And in the car, while I would turn on the AC, I would definitely not like it to continuously blow in my face, I don't like the feeling and it dries out my eyes (and with wearing contacts that's really uncomfortable). In the summer, I think most German houses do not really need AC, as the insulation actually works, and it doesn't heat up as easily. I lived in Oklahoma for a year and there, the heat wouldn't have been bearable without air conditioning, but here I only miss it on the busses and in stores, especially clothes stores. Who likes trying on clothes when they are all sticky?!
About keys in every lock: not sure what the reason is, but every now and then I do lock myself into my room if I don't want to be disturbed. They can also be helpful at Christmas when the Christkind puts the presents under the tree and doesn't want to be caught.
I never meet people around lunch time that say Mahlzeit to me, unless I am sitting down to have lunch with them, and before we start eating we say that or "guten (Appetit").
I have screens in front of my windows so they do exist. Admitted they are only nets that stick to the frame with velcro, but it works just as well.
About not saying Happy birthday before the actual birthday. That's for the same reason there are often no 13th floor/row in buildings/planes... of course it is superstition but superstition is not a German thing. We just don't do it, just like we don't ask everyone how they are doing, it's a German thing. Not saying one thing or the other is better, they're just different.
About washing machines and dryers: I have never used a toploader washing machine but I have heard from many Germans who live in the US (and physically it makes sense) that they do not clean the clothes as well. About dryers: I have one here, and I love putting on warm clothes that just came out of the dryer, or using a still-warm towel, but back home (at my parents') we don't have one, and we have a room that just keeps the oil tank/heating so we hang up our laundry there, and that works just well. Here I would not want to put up my laundry all over my apartment.
But try living in Spain, they hang their clothes outside, not sure what they do in the winter, but I know when I was there, and it rained I had to hang it on hangers on doorknobs window handles, everything because obviously I couldn't put it outside.
About the clapping after a plane lands, I kinda always thought that was an American thing, and that some Germans did it to be "cool". Because we all wanna be all American as ya know.

About the clapping with your knuckles on the table, I first encountered that during a lecture at uni, but now that I've been in uni 4.5 years I just do it, and I do not even feel ridiculous anymore; it becomes a habit after a while.
And about that remark on Mexicans: the term Mexicans is not used just for Mexicans in the US but for anyone looking Mexican (i.e. a Hispanic person with Indio roots). Now Hispanic is not a race, but an ethnicity but it is still considered racism when you say something hateful and negatively biased about them.
Just thought I'd put in my $.02 (or €.02?

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This is all coming from a Munich born and raised girl who does not speak Bavarian, does not like beer, and does not like skiing.