What boggles my mind (aside from the obvious) is that they all pick on each other about whose house is worse.
"Oh I know mine is bad, but you should see ______ house!"
I always want to tell them that just because your mom or whoever has a house full of crap doesn't make your house clean by default.
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Funny. After she was forced to move to an apartment, my SIL started giving me tips on how to clean, and actually ran her fingers over the furniture to check for dust. We stopped going to her house for holidays, because it was so filthy. Among other things, the dog isn't housebroken.