I notice that HN automatically removes some words from titles when they're pasted in. I've never been clear on what words are removed, and it often feels random

I notice that HN automatically removes some words from titles when they're pasted in. I've never been clear on what words are removed, and it often feels random. I don't think this is the actual reason, but it almost feels like it's done to see if the poster is paying attention? Either way, I've always been able to edit the title to bring the words back if they're key to the post's meaning.