Ok you're probably thinking what does an old peanut butter jar filled with rock salt have to do with the fact that at the moment it is so hot in Australia that you can fry eggs on your porch... the answer is everything.
This time last year I send James to go and buy me a dehumidifier (he accidentally brought a diffuser and I then had to go and buy a dehumidifier, but I'm not going to go there) not because we were getting mould in our bathroom (although that is also a miracle because we have no window in there, I think we have anti-mould paint) but because we were getting that weird drippy wall thing.
Because I have boycotted the whole plastic thing and also because the chemicals used in the dehumidifier beads are awful I decided to research an alternative. In winter I had this jar chilling in my bathroom and I didn't really notice any water on the walls, but it was winter and I wasn't expecting it. But now that it is stinking hot and it feels like you're walking through hot water outside at the moment, when I came home this afternoon I was expecting the drips to be there still, especially since James only showered at 1pm (he's on a late shift). Instead I found no drips but residual water in the jar.
Salt can be dissolved in water and therefore salt absorbs water droplets. Simple, Cheap as chips, zero waste, zero plastic, completely natural, lasts for ages, etc etc