The idea of glass that cleans itself might seem like something out of science fiction, but the technology has actually been around for more than a decade.  It works via a coating of titanium dioxide, which is used in a range of unusual places – from white paint and food colouring to suntan lotion.  When it’s placed on windows, it’ll respond to sunlight by transforming surrounding water molecules into hydroxl radicals, which will break down any surrounding organic matter (like the microscopic gunk from which dirt is made).  Once broken down, these tiny quantities of dirt are easily washed away by rainwater.