Humans can see water but not air. So can fish see air? And would it be possible to have anti-black holes, and how could we tell if we were looking at one? https://www.newscientist.com/article/2583807-this-weeks-new-questions-358/?utm_term=Autofeed&utm_campaign=echobox&utm_medium=social&utm_source=Twitter#Echobox=1787059764