How is a mood injury significantly different from a skin or tendon injury?

You need your mind and your body for work.

If you injure one it may cause a small amount of impairment but not necessarily enough to prevent you from working.

However if someone has a 'thing' about limps and sees a limping man as a distraction that slight malfunction in one worker triggers a concentration malfunction in the other.

If they worked on their attitude to limps so it wasn't a 'thing' anymore they guy with the wobbly foot and the guy who used to be hung up on limps both can get through the day and pick up their wage check without having to resort to ugly bloody violence.

Similarly one worker may have been mentally traumatised before coming to work.

Maybe her cat was run over by a slow moving car or his wife had foolishly allowed herself to be caught their bed with another man.

This is the limp but in a mental sense.

They can still work but they have a bit of a short fuse (nothing fatal) but enough to annoy someone who finds short fuses distracting enough for it to be a 'thing'.

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