Mum is repairing a hemline on a dress.  Unfortunately she is using invisible thread.  So called because you can’t see it.  She is having problems with an invisible knot.  Aunty Biscuit has a spool of invisible thread at her house that she can’t find.  Surely the clue is in the name???  I suppose that the advantages are that if stitching is a little uneven it won’t show and it does for all colours.  As it looks a bit like very thin fishing line I suspect it is also very strong.