I told my teenage niece to go get me a newspaper...

I told my teenage niece to go get me a newspaper...
She laughed at me, and said, "Oh uncle you're so old. Just use my phone."
So I slammed her phone against the wall to kill a spider.

This is a joke about a deliberate misunderstanding. The jokester in this joke probably knows that the niece didn't expect him to smash a spider with the phone, but he does it anyway. Misunderstandings are funny when they happen to other people because we are glad they are not happening to us.

The fact that he has such poor impulse control is also funny - honestly who would do that?