It's not rocket science

This is one of my favorite (BrEng favourite), and often used, English expressions. When you want to say that something is not as difficult or complicated as people might think, or that someone is acting like something is difficult or complicated, you can say "It's not rocket science." The idea is that rocket science is indeed difficult, and that what you're talking about is simple in comparison.

This is a very common expression in English (304,000+ hits on Google), for example:

Developing Your Project Doesn't Have to Be Rocket Science

Financial management is not rocket science

It's Not Rocket Science is Captain Everything's third full length album