Dic: work on something

• He's out in the kitchen, working upon his tax forms.
• The carpenter worked on the fence for three hours..
• This medicine should work well upon your cold.
• I hope it will work on your cold.

1. to repair or tinker with something. | to repair, build, or adjust something.

2. [for something] to have the desired effect on something.

(Upon is formal and less commonly used than on.)

... http://idioms.thefreedictionary.com/work+on

CF:

VERB | If you work on an assumption or idea, you act as if it were true or base other ideas on it, until you have more information.

• We are working on the assumption that it was a gas explosion.
  

... Cobuild

work on (Also, work upon): Exercise influence on:

• If you work on him, he might change his mind.
• 'She always worked upon their feelings by pretending to be more ill than she really was. [Early 1600s]
  

... Am Heritage Dic of Idioms

work on:

• Meaning #1: to exert effort in order to do, make, or perform something(= work at):
  she worked at it until she had it down cold.
  
• Meaning #2: shape, form, or improve a material (= work, process)

... http://www.thefreedictionary.com/work+on