In the early 1950s Bette Nesmith Graham was a secretary at a bank in Dallas, not a brilliant typist by her own account, working on the new electric machines that smudged whenever you tried to erase. She noticed that the artists painting the bank’s holiday windows never rubbed out a mistake: they painted over it and carried on. So she put some white tempera paint in a little bottle, took a watercolour brush to the office, and quietly painted over her typos for years before she sold it as Mistake Out, later Liquid Paper. What I like is that it never removed anything. Every letter corrected that way is still on the page, the wrong one, under a thin white coat, and if you hold one of those old letters up to a window you can often read them all: a page that looks perfect, with its whole history underneath.