SOLID principles — tell me about each one.

SOLID is a set of five principles of object-oriented programming that help create flexible and maintainable systems:

1. S — Single Responsibility Principle A class should have only one reason to change, meaning it should perform only one task.

2. O — Open/Closed Principle Software entities should be open for extension but closed for modification.

3. L — Liskov Substitution Principle Subtypes must be substitutable for their base types without altering the correctness of the program.

4. I — Interface Segregation Principle Clients should not depend on interfaces they do not use. It's better to have several specific interfaces than a single general one.

5. D — Dependency Inversion Principle High-level modules should not depend on low-level modules. Both should depend on abstractions (interfaces).

These principles help write code that is easier to test, extend, and maintain.