# Parable of the vanilla ice cream curse (and how it would prevent a car from starting!)

Story from [https://www.cs.cmu.edu/~wkw/humour/carproblems.txt](https://www.cs.cmu.edu/~wkw/humour/carproblems.txt) (my thoughts follow):

> For those of us who understand that the obvious is not  
> always the solution, and that the facts, no matter how implausible,  
> are still the facts ...
> 
> A complaint was received by the Pontiac Division of General Motors:
> 
> "This is the second time I have written you, and I don't blame you  
> for not answering me, because I kind of sounded crazy, but it is a  
> fact that we have a tradition in our family of ice cream for dessert  
> after dinner each night. But the kind of ice cream varies so, every  
> night, after we've eaten, the whole family votes on which kind of ice  
> cream we should have and I drive down to the store to get it. It's  
> also a fact that I recently purchased a new Pontiac and since then my  
> trips to the store have created a problem. You see, every time I buy  
> vanilla ice cream, when I start back from the store my car won't  
> start. If I get any other kind of ice cream, the car starts just  
> fine. I want you to know I'm serious about this question, no matter  
> how silly it sounds: 'What is there about a Pontiac that makes it not  
> start when I get vanilla ice cream, and easy to start whenever I get  
> any other kind?'"
> 
> The Pontiac President was understandably skeptical about the letter,  
> but sent an engineer to check it out anyway. The latter was surprised  
> to be greeted by a successful, obviously well-educated man in a fine  
> neighborhood. He had arranged to meet the man just after dinner time,  
> so the two hopped into the car and drove to the ice cream store. It  
> was vanilla ice cream that night and, sure enough, after they came  
> back to the car, it wouldn't start.
> 
> The engineer returned for three more nights. The first night, the  
> man got chocolate. The car started. The second night, he got  
> strawberry. The car started. The third night he ordered vanilla.  
> The car failed to start.
> 
> Now the engineer, being a logical man, refused to believe that this  
> man's car was allergic to vanilla ice cream. He arranged, therefore,  
> to continue his visits for as long as it took to solve the  
> problem. And toward this end he began to take notes: he jotted down  
> all sorts of data, time of day, type of gas used, time to drive back  
> and forth, etc.
> 
> In a short time, he had a clue: the man took less time to buy  
> vanilla than any other flavor. Why? The answer was in the layout of  
> the store.
> 
> Vanilla, being the most popular flavor, was in a separate case at  
> the front of the store for quick pickup. All the other flavors were  
> kept in the back of the store at a different counter where it took  
> considerably longer to find the flavor and get checked out.
> 
> Now the question for the engineer was why the car wouldn't start  
> when it took less time. Once time became the problem -- not the  
> vanilla ice cream -- the engineer quickly came up with the answer:  
> vapor lock. It was happening every night, but the extra time taken to  
> get the other flavors allowed the engine to cool down sufficiently to  
> start. When the man got vanilla, the engine was still too hot for the  
> vapor lock to dissipate.
> 
> Moral of the story: even insane-looking problems are sometimes real.

There are times when people tell me that (metaphorically) their car won't start because of the vanilla ice cream spell.

I usually assign rather low probability to their claims that (metaphorically) "vanilla ice cream causes their car to not start".

But I *also* would still assign very low probability to the "vanilla ice cream spell" explanation of the causation *even if* I believed a lot of scientifically sound data was collected to support the causal link between "buying ice cream" and "car won't start".

I'm not "refusing the evidence" by not believing in the vanilla ice cream spell hypothesis. The data just says there's a causal link—it doesn't say why. Jumping to the spell hypothesis disregards everything else we know about physics—it's more likely that there's a more reasonable explanation we didn't think of. By reasonable, I mean an explanation that would add up to normality—that wouldn't require us to update our would model nearly as much.