What kind of things can’t I protect as a trade secret?

Anything which has been publicly disclosed, and which is “out there” in the public sphere, cannot be protected as a trade secret. Even if something is secret, however, it may not be protectable as a trade secret if it does not offer its owner any value through its secrecy. For example, if you create the idea of a machine capable of time travel, but have no real way of implementing that idea, it does not offer any economic value to you and so it is not entitled to trade secret protection. Of course, if you actually design such a machine, such as the flux capacitor, then you would be entitled to trade secret protection (as long as you don’t disclose the machine’s secrets in a trilogy of 80’s movies).