Hypothesis Jokes

A mathematician has spent years trying to prove the Riemann hypothesis - without success. Finally, he decides to sell his soul to the devil in exchange for a proof. The devil promises to deliver a proof within four weeks.
Four weeks pass, but nothing happens. Half a year later, the devil shows up again - in a rather gloomy mood.
"I'm sorry", he says. "I couldn't prove the Riemann hypothesis either. But" - and his face lightens up - "I think I found a really interesting lemma..."

A mathematician, a physicist, and an engineer are asked to test the following hypothesis: All odd numbers greater than one are prime.
The mathematician: "Three is a prime, five is a prime, seven is a prime, but nine is not a prime. Therefore, the hypothesis is false."
The physicist: "Three is a prime, five is a prime, seven is a prime, nine is not a prime, eleven is a prime, and thirteen is a prime. Hence, five out of six experiments support the hypothesis. It must be true."
The engineer: "Three is a prime, five's a prime, seven's a prime, nine's a prime..."

A mathematician has been invited to speak at a conference. His talk is announced as

Proof of the Riemann hypothesis.

When the conference actually takes place, he speaks about something completely different.
After his talk, a colleague asks him: "Did you find an error in your proof?"
He replies: "No - I never had one."
"But why did you make this announcement?"
"That's my standard precaution - in case I die on my way to the conference..."