Thoughts are Objects that Strike. Knowing is Possessing.

Description of motion:  Hand points to head.  (In this particular instance, the hand was holding a pen, so the pen acted as a finger.)
Observed: Rad School meeting.  1-14-2015.  13 people seated around a rectangular table.
Example:  "That song has been stuck in my head all day."
Link to use:  Why do songs get stuck in my head?
Use:  Used to describe what it feels like to be unable to stop thinking about something.
Metaphor: Thoughts are Objects that Strike.  Knowing is Possessing.
Explanation:  If Thoughts are Objects that Strike, then a thought that has become "stuck" is a thought that has hit you particularly hard.  Because of the force of the impact, the thought has become wedged in your brain (Knowing is Possessing).  It stays there unable to move, and so you are unable to think about anything else, that is, until you find a way to get it "unstuck".  What is implied in this series of metaphors is that there is a fluid like motion to thoughts.  They come and they go.  It is somewhat irregular when a thought does not leave, and depending on how it "strikes you" the thought might wedge itself in a position where it impedes the natural fluid movement of thought.

Origin:  Our physical experiences with getting spears stuck in animals, ourselves stuck in the mud, rocks wedged between other rocks, would give us access to think about abstract ideas as also having the ability to wedge themselves, to be stuck, to be unable to move.  An idea that we are unable to let go of, that sticks with us, that we cannot shake, would then be relatable back to this early experience of a physical object stopping our motion. 

Notes:  Initially I was confused over two differing uses of the word stuck.  The first is the way I've described above such as "having a thought stuck in your head."  The other way we use stuck is when we say something like, "I've been stuck on level 3-1 of Super Mario Bros for months."  The main difference between the two is that "stuck on a problem" implies a whole grouping of coherent metaphors, the first one being Distance is Progress, in that, there is a distance that you have to go before you solve the problem.  

In the drawing above you can see that there are a number of implied metaphors at work here that fall under the overall metaphor of Problems are Journey's.  Like most journeys there is a destination, a path, a beginning and an end.  The path can be uphill or downhill.  It can be windy or straight.  The distance can be long or short.  The force can be weak or hard.  This series of coherent metaphors make sense when we say something like, "I'm so close to finishing this paper.  (Distance is Progress)  I've been pushing myself really hard (Force is Determination) and now I'm really on a roll.  (Momentum)  It's all down hill from here. (Path) "   Because the path is described as "downhill" and since you are "pushing really hard" you roll easily down the hill and pick up speed.  Because you have picked up so much speed any obstacle in your way is easily rolled over because of the momentum that you've built up.  The above diagram is perfectly captured in the saying, "Let's get the ball rolling." 

Having a thought stuck in your head, does not imply a journey of any kind.  It simply points toward the overall nature of thought as moving or flowing, but not in any sort of way that has an end point, a key characteristic of a journey.  Since in this instance, distance is not related to progress, what you are left with is the task of letting go, or shaking off the thought that is stuck, so that thoughts can resume their normal flow.