I couldn't find it because from memory I was googling 'feynman furry'. The keywords are 'feynman "grow hairs"'. From his book called "Surely You’re Joking, Mr Feynman":
"I can't understand anything in general unless I'm carrying along in my mind a specific example and watching it go. Some people think in the beginning that I'm kind of slow and I don't understand the problem, because I ask a lot of these "dumb" questions: "Is a cathode plus or minus? Is an an-ion this way, or that way?" But later, when the guy's in the middle of a bunch of equations, he'll say something and I'll say, "Wait a minute! There’s an error! That can't be right!" The guy looks at his equations, and sure enough, after a while, he finds the mistake and wonders, "How the hell did this guy, who hardly understood at the beginning, find that mistake in the mess of all these equations?" He thinks I'm following the steps mathematically, but that's not what I'm doing. I have the specific, physical example of what he's trying to analyze, and I know from instinct and experience the properties of the thing. So when the equation says it should behave so-and-so, and I know that's the wrong way around, I jump up and say, "Wait! There’s a mistake!"
Also from the same book:
"I had a scheme, which I still use today when somebody is explaining something that I'm trying to understand: I keep making up examples. 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!'"
"I can't understand anything in general unless I'm carrying along in my mind a specific example and watching it go. Some people think in the beginning that I'm kind of slow and I don't understand the problem, because I ask a lot of these "dumb" questions: "Is a cathode plus or minus? Is an an-ion this way, or that way?" But later, when the guy's in the middle of a bunch of equations, he'll say something and I'll say, "Wait a minute! There’s an error! That can't be right!" The guy looks at his equations, and sure enough, after a while, he finds the mistake and wonders, "How the hell did this guy, who hardly understood at the beginning, find that mistake in the mess of all these equations?" He thinks I'm following the steps mathematically, but that's not what I'm doing. I have the specific, physical example of what he's trying to analyze, and I know from instinct and experience the properties of the thing. So when the equation says it should behave so-and-so, and I know that's the wrong way around, I jump up and say, "Wait! There’s a mistake!"
Also from the same book:
"I had a scheme, which I still use today when somebody is explaining something that I'm trying to understand: I keep making up examples. 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!'"