[woofie]

How do you prove a negative?

[Dog]

Find a source grasshopper.

[viiince]

How do you prove a negative?

Ha! That's what Saddam said, too.

[Southack]

"How do you prove a negative?"

By exhausting all other possibilities in a set.

If someone says that "it's not 9" and you have a set of answers from 1 to 10, then you simply plug in the numbers 1,2,3,4,5,6,7,8,10. If those numbers turn out negative, then it is 9, otherwise it isn't.

Then you've just proved or disproved a negative ("not 9").