Knitting is something that people have been doing for centuries to create garments that would keep them warm.
However, the Office of Censorship felt forced to step in and control the patterns knitters used during the Second World War, a fascinating new book reveals.
John Lloyd, John Mitchinson and James Harkin have penned 1,339 QI Facts to Make Your Jaw Drop - an extract of which is published in the Telegraph - and they state that it was forbidden to post knitting patterns abroad in case they contained coded messages for the enemy.
This may seem preposterous, but it seems coded knitting did actually happen - albeit only once. In Belgium, resistance fighters asked old women to sit at their windows and knit while they overlooked railway yards.
If a certain type of train went by, they had to purl one. If another chuffed past, they would drop a stitch, presumably giving the rebels a chance to learn about logistics movements they could then try to disrupt.
Lloyd, Mitchinson and Harkin suggest they might have got idea from Charles Dickens's A Tale of Two Cities, in which Madame Defarge knits the names of French revolutionaries as they go to the guillotine.
Of course, the government encouraged Britons to knit during the war more than they imposed restrictions, as woolly garments were essential for the troops.
The slogan 'If You Can Knit - You Can Do Your Bit' encouraged everyone to pick up their needles and craft pullovers, vests, gloves, balaclavas and much more for the Army, Navy and Air Force. Specially dyed yarn of khaki, navy blue and grey was even produced to keep up with the demand and ensure the troops who used the garments would remain camouflaged or perfectly coordinated with the rest of their uniform.
Those days may be gone, but we love the idea of hiding secret messages in our knitting - perhaps you could try your initials at the bottom of a cardigan, or a heart in a sweater for a beloved grandchild?
Let us know what you hide and if you've got any more ideas!