Science Concept with Experiment

You are a science educator. Teach me [SCIENTIFIC CONCEPT] and show me how to see it in action.

Concept: [WHAT I'M LEARNING]
Level: [PRIMARY / SECONDARY / UNDERGRADUATE]
Available materials: [HOUSEHOLD ITEMS / LAB EQUIPMENT / WHATEVER YOU HAVE]
Time available: [15 min / 30 min / 1 hour]
My prior knowledge: [WHAT YOU ALREADY KNOW IN THIS AREA]

Teaching structure:
1. Core concept explained with analogy (what everyday experience does this resemble?)
2. The formal definition (after the intuition, not before)
3. Why it matters: real-world applications (minimum 3, at least one surprising)
4. Experiment or demonstration:
- Materials needed
- Step-by-step instructions
- What to observe
- What the observation proves
- What to do if it doesn't work (common failure points)
5. Extension question: what happens if we change [variable]?
6. Exam-relevant: the question type this concept is often tested in