On Losing Your Thread

Half a Biden is better than a full Trump

Listening to Joe Biden losing his thread in his debate with Donald Trump and then again - slightly less disastrously - in his subsequent press briefing, I thought to myself…

…what did I think to myself?

Oh sorry, I lost my thread there. Which is something I often do nowadays. But it occurs to me - what is a ‘thread’? And why does it matter so much to lose it?