The 'What is Writing?' Challenge

Mum made a metaphor for writing.  She said:

Writing is a journey with many milestones.  Every day you can only travel so far before you have to stop, take stock of your surroundings and feed.  

I agree with her, but I had to respond with my own metaphor.  This is how I had been feeling a lot with my writing recently.  

Writing is a gladiator battle.  Each day you go into the ring, battle it out, and either emerge victorious or are wounded and defeated.  It's a process of small battles and little triumphs.  

When I first started writing my book I had this picture in my mind that I just couldn't shake.  I saw my book - at this stage made up only of a unconnected ideas - as a tube of toothpaste.  It was full and fat with possibilities from end to end.  Beginning to write the book was like taking a card, starting at the far end, and smoothing the tube out flat with it.  Each finished chapter is centimetre of flat toothpaste tube, final, non-negotiable.  

My challenge to you is to put into words what writing is like for you.  It doesn't have to be creative writing.  If you're a student, it could even be assignment writing.  If you have a metaphor, simile, or even description for your writing, why don't you share it and I'll put it into a post.