On topic: things people can run on their computers or hold in their hands. For hardware, you can post a video or detailed article. For books, a sample chapter is ok.
Off topic: blog posts, sign-up pages, newsletters, lists, and other reading material. Those can't be tried out, so can't be Show HNs. Make a regular submission instead.
Things that can't be Show HN can be perfectly fine regular HN posts, just like this thing already is.
I don't know, 0 posts and two comments, first post with self promotion right away. I had a feeling it's not allowed, oh well
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Just look at their "Projects" page here: https://x.company/projects/ Still touting public failures like Everyday Robots, Loon, Glass, Makani, now Mineral. Fail fast shouldn't mean fail always...