How can a girl reply humorously when she says you can't?

I think people should learn from difficulties. When friends say you can't do it, I think you should bite the bullet. I think you should. Prove to her that you are actually capable and you can totally overcome it. As long as I persist, I feel that only when I am oppressed will I have motivation. Only when others belittle you blindly will you make a decision to fight back. I think you have to prove to him that you are fully capable.