Rotting design and viscosity

Question

Robert C. Martin in one of his book mention about viscosity as a symptom of rotting design. I don't fully understand idea of viscosity in software development. Could you give me some examples?

Answer 1

As code gets older, it gets more full of hacks, and more full of dependencies, and consequently gets harder to change without breaking something.

So it gets less "fluid" and more "solid", hence the analogy to viscosity.