It’s the “hairy green ball” from “Surely you are joking...”:
For instance, the mathematicians would come in
with a terrific theorem, and they’re all excited. As they’re telling me the conditions of the
theorem, I construct something which fits all the conditions. You know, you have a set
(one ball) disjoint
(two balls). Then the balls turn colors, grow hairs, or whatever, in
my head as they put more conditions on. Finally they state the theorem, which is some
dumb thing about the ball which isn’t true for my hairy green ball thing, so I say, “False!”
If it’s true, they get all excited, and I let them go on for a while. Then I point out
my counterexample.
It’s the “hairy green ball” from “Surely you are joking...”: