system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is owning 5% of a company such a big deal for hostile takeovers?

I've seen on a few shows now (Suits, Billions) that owning 5% percent of a company is a huge first step in taking over/making a tender offer for a company. Why is this such an important milestone?
gpt
Anyone who owns 5% of a public company has to register a schedule 13D with the SEC, which means their ownership is now public knowledge.  If you plan a hostile takeover the element of surprise is over when you cross the 5% mark.