What Keeps A Twister Spinning?

So , I was watching the movie Twister and it was at the part where they were in the drive-in and I was wondering how does the twister keep spinning , how does it even start? With wind? Or does it start with a storm. I mean it's just confusing because wind doesn't just randomly affect one spot and just create a circle of wind , And the clouds don't just randomly start to turn. So how does a twister start? And how does it keep spinning?