@Shamar
The joke is that doubling half of something should be the same as multiplying by 1. 'Mathematician' here is just another name for 'a numerate person'.
But there is some work done in crochet and maths, e.g. http://www.toroidalsnark.net/mathknit.html
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If you double half square (cut over a diagonal) you might get the original square, a triangle or a parallelogram: it all depend on how you arrange the two halves you get.
If you multiply one square by one, you can only get a square.
So I think I got the joke, I was just surprised to see mentioning #crochet in #math, a concept I've never heard before.
@Shamar
Thanks @koantig, couldn't have said it better myself.
As a bonus, here is the comparative height of #crochet stitches.