It’s not rocket science is commonly used to say that something is not very difficult to do or understand.
Designing a website may be a lot of work but it’s not rocket science.
The expression comes from the States.
After the end of World War II a group of military scientists from Germany, who’d been working on the new technology, were transferred to America and contributed greatly to the success of the U.S. space programme.
The phrase was born not only out of the understanding of how much knowledge is required to build a rocket, but also Germany’s reputation as a society of well-educated and cultured individuals, which the recent war tarnished but didn’t ruin completely in public consciousness.
The image of Albert Einstein, an acknowledged scientific genius from Germany who was working in America at the time, might have also inadvertently strengthened this popular belief.