Rules of Computer Science

As I've spent these many years learning and practicing computer science, I've come to have a couple of "rules" (i.e. guidelines) of programming.

1) If a computer can execute something, you can write it in C. (aka Turing-completeness)

1a) Still, it is probably faster to write the program in a high-level language.

2) The computer does what it is told to do. There is no magic.

2a) If a behavior is not what you expect, then you don't know what you told the computer to do.

I am sure there are other such "rules", so please share if you have other maxims from your time and experience.