I'm thinking of stealing my boss's pens
Are you really sure? Why would you want to do that?
I need some pens and my boss has quite a few.
It is wrong to steal from people especially your boss. Employers are supposed to provide work materials to their employees and you should ask them.
Why do you have to be so rigid? It's only a pen.
Even thought it's something so small doesn't mean you can just take it. It still someone else's property, and small things can snowball into larger things. Why not ask your boss to give you some pens, I'm sure he wouldn't mind.