Believer: Worship the invisible pink unicorn.
Atheist: Sorry, I don't believe he exists.
Believer: Prove it.
Atheist: Huh?? Why should *I* come up with proof? Ok, I can try: I don't see him.
Believer: That is because he is invisible. Also he is pink. That is one of his divine properties.
Atheist: I don't hear him.
Believer: He only speak to believers. I hear him answer when I pray to him, I'm convinced of that.
Atheist: Ok, here I have an infrared camera. I don't see anything.
Believer: He doesn't emit heat.
Atheist: Ok, I throw around flour and see if anything stick to him, or if we see any footprints appearing.
Believer: Sorry, he is immaterial.
Atheist: Ok, what is the difference between a totally undetectable creature and one that doesn't exist?
Believer: When rain falls, the invisible pink unicorn caused it, whenever a child laughs, the IPU caused it. Also 6000 years of belief shows I am right.