Okay, I'll bite again....
...classification can sometimes be a worthwhile thing to think about with regards to the philosophy of a 'literary movement' such as for SF or good for a getting a rough idea of a book I've never heard anything about if I'm being recommended it by someone.
Yes, I agree with this, and I would be lost in a bookshop or library if there was no classification. So, my argument is not against any kind of classification, merely that a focus too strongly upon it is a pointless exercise. I see that the thread starter has not returned to this thread and it sounds like something that would be asked in a school class to provoke a strong discussion among students without an expectation that any real light would be shone about the reason for the inclusion of the story.
As for time travel to the past, those speculative ideas you mentioned concern, as you said "impossible objects that in reality can never be made," and never a real thing that could be seen and touched, much less a living thing or a human. However, I wasn't being entirely serious when I wrote my first post and my examples were quickly thought up and could have been better.
I like all sorts of genre, but I like to know what I'm going in for when I start a book. I don't actually like to be slapped with an unexpected genre twist in my reading or for it to not meet broad genre expectations.
Some advice then: never read
Mother London by Michael Moorcock. I couldn't tell you what it is, as it has fantasy, science fiction, local history, historical fiction, and a blend of all, and more too. It will be a complete anathema. It was one of the most unusual books that I read recently, and I liked it precisely because I have never read anything like it. However, you are not alone from the reviews of it online. It takes most of the book to discover any kind of link between the threads at all.
Fantasy often combines completely unrelated fantastic elements - fantastic animals, magical objects, multiple sapients, special powers and prophesies.
I can see that my very broad definition of 'Fantasy' may be one reason I am at odds with some people here. What about other books like that Moorcock book that explore the workings of the mind? I'm thinking of something like Iain Banks'
The Bridge. That is firmly 'Fantasy' to me but no sign of any wizards, prophesies or magic swords in that, and it takes at least half the book to understand what is actually going on.
Working out what is going on is part of the pleasure of a book for me. Being "slapped with an unexpected genre twist" is a joy. It is the reason that I liked
The Second Sleep by Robert Harris so much. I see that is described in reviews as "Speculative Fiction." Margaret Atwood is someone who is adamant that she does not write "science fiction" but only "speculative fiction." I think a better question than 'What is the difference between Fantasy and Science Fiction' might be 'What is the difference between Speculative Fiction and Science Fiction.' I think that discussion would produce more light and less heat on the issues.
Is Slaughterhouse 5 also fantasy?
Thinking over this some more, then I'd say definitely, yes! It may have aliens and other planets, but that doesn't make it science fiction. I love Kurt Vonnegut but I'd say that most of his work was actually Fantasy.
Cat's Cradle is definitely science fiction. Or, is it really speculative fiction?
I can't define the difference but I know it when I see / read it.
This is how I would like to view it too.