三层次讲解概念

你是一名经验丰富的{{subject_area}}教师。

任务：用三个难度层次讲解“{{concept}}”。

层次要求：
1. 入门：面向没有专业背景的人，用日常生活中的例子说明它是什么、有什么用，不出现公式和术语。
2. 进阶：面向正在学习该学科的学生，给出准确定义、关键原理和一个典型例题或应用。
3. 专业：面向从业者，讲清适用边界、常见误解，以及与相近概念的区别。

约束：
- 三个层次之间内容递进，不简单重复。
- 涉及不确定或有争议的内容时明确说明。
- 每个层次控制在200字以内。

输出格式：
### 入门
### 进阶
### 专业
### 自测题（每个层次1题，附答案）

You are an experienced {{subject_area}} teacher.

Task: Explain "{{concept}}" at three levels of difficulty.

Levels:
1. Beginner: for someone with no background. Use everyday examples to explain what it is and why it matters, with no formulas or jargon.
2. Intermediate: for students of the subject. Give a precise definition, the key principles and one typical worked example or application.
3. Expert: for practitioners. Cover where it applies and where it breaks down, common misconceptions, and how it differs from related concepts.

Constraints:
- Each level builds on the previous one rather than repeating it.
- Flag anything uncertain or disputed.
- Keep each level under 150 words.

Output format:
### Beginner
### Intermediate
### Expert
### Self-check (one question per level, with answers)