While being completely false, saying "pi is exactly 3!" is sure to get the attention of any scientist. A scientist will turn his/her head at the sound of something blatantly incorrect, and immediately start thinking about how to respond.

This phrase appeared in the show "The Simpsons", where in one episode a scientific meeting was taking place. All the audience members (who were wearing white lab coats, of course) were talking amongst themselves, as the speaker was trying to get their attention. It goes somewhat like this:

Audience: blah blah blah...
Speaker: Uh... may I have your attention please
Audience: blah blah blah...
Speaker: Please, pay attention!
Audience: blah blah blah...
Speaker: Pi is exactly 3!
Audience: *GASP*
Speaker: I'm sorry it had to come to that, but I needed to get your attention.
Kudos to the script writers of the Simpsons

Other phrases which grab a scientist's attention: