How original do stories need to be?

The other night I was writing a story in bed.

In my mind. It's what I often do.

I lie in bed and I just create the story itself. The next day, if I like the story, I may begin to actually work on it.

Sometimes, I will forget the story very quickly if I don't get it out of my head.

Other times, it will stay with me for a long time.

When it stays with me, I often do end up writing it.

This story was about a young boy who gets transported from his swimming pool into another world.

It's a horror story. A very short story. I usually prefer the very short stories even though I have been writing longer ones lately.

It made me think about something I'd seen on TV before though.

This is not a completely original idea.

However, I like horror stories.

I like stories about spaceships.

I've seen many episodes of TV shows which were almost exactly the same as other episodes. I liked them though.

Time traveling episodes. It doesn't matter that the same thing happens twice. It has different characters and different scenarios.

So, I wonder how original stories actually need to be?

I think they really just need to be entertaining.

They shouldn't be duplicates. That's for sure.

However, if they're entertaining, it probably it doesn't matter.