Sarah Miller got me thinking about inertia.
A body at rest tends to stay at rest, and a body in motion tends to stay in motion until they are acted upon by an external force.
This is the story of my writing life!
When writing is moving along, it feeds itself. I get new ideas all the time, even (especially) when I'm supposed to be doing something else. When my revising stalls, a huge force is required to get it going again.