Conceptualize a very basic software system and develop a high-level object-oriented design for it using UML diagrams. Follow the steps below:

Step 1: System Conceptualization
Imagine a simple system like a Calculator. Briefly describe the system's main functionalities and operations.
Next, identify and describe the key components or classes of your system and their responsibilities.

Step 2: UML Diagram Creation
Based on the system conceptualized in Step 1, create the following UML diagrams:
Use Case Diagram: Identify the main actors and their interactions with the system.
Class Diagram: Present the primary classes, their attributes, and methods.

Deliverables:
A brief description of your imagined system and its main components.
Your Use Case and Class Diagrams with appropriate captions and short descriptions.