Something fishy about these beliefs.

If Julie has a fault, it lies in not letting go of an idea once it has taken root, even when evidence to the contrary is provided. In a less-intelligent person (and Julie is intelligent and quick-witted), this would mean being unable to count up to ten - or perhaps being able to count up to thirteen. And a propensity for playing the banjo.

The main problem is Julie gets lost with details of new stuff, and I'm not that good at explaining things. You may have noticed this.
For example, take this occasion. We were in a cafe and talking about food in general. Like any couple, there are things that one likes that the other one dislikes. I like Marmite; Julie doesn't. Julie loves ketchup; I can't stand it.

The big one is seafood. In the main, I absolutely love it, and if I haven't tasted it, then I'll at least give it a try. Julie, on the other hand, is very much a white fish person and only likes prawns if they come with lettuce and 1000 island dressing.
Unfortunately, things that live in the sea suffer from Julie's tendency to stick with certain notions. In this case, it's that everything in the sea is a fish.

Cue lengthy, involved diatribe on what a fish is - and isn't. All followed up with this.

Well, 'fair enough', I laughed - and then launched back into my attempts at education. When we got to crustaceans - specifically crabs - Julie began talking animatedly about how she couldn't stand them. When I asked her what crabs were, she replied that they were 'sea creatures'.
I pointed out that she had just contradicted her earlier comment about everything in the sea being 'fish', and she then uttered the following Gem:

I gave up at that point.