| This is the first question I was asked in your first theory of knowledge lesson for the IB. It's a good way of confusing people to such a degree that they will pretty much listen to anything you have to say in hope of regaining their sanity.
The argument goes roughly like this:
Teacher:- Is that table really there?
Student:- Of course it is.
Teacher:- How do you know?
Student:- Because I can see it.
Teacher:- How do you know your seeing it?
Student:- Well.....errrm.....Because I have the image in my brain?
Teacher:- Is that image real?
Student:- Of course it is!
Teacher:- Prove it.
Student:- Errrm....I know the table's there because I can touch it.
Teacher:- How do you know your touching it?
Student:- Haven't we done this before?
Teacher:- How do you know?
Student:- AAAARRRRRGGGGHHHH
You get the idea. | 
