First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic of total internal reflection. It does not need other sentences for context. It gives a broad overview of what will be discussed. Why other lines are not chosen? Paragraph 2 starts with "To demonstrate" which is a linking phrase, so it cannot be first. Paragraph 3 starts with "When the light travels" which needs context. Paragraph 4 starts with "Even as the water flow" which also needs context.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the explanation of total internal reflection. It shows the final result of the experiment, which is the laser beam disappearing.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it uses the connector "To demonstrate". This shows that the video will explain the total internal reflection mentioned in Paragraph 1.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 follows Paragraph 2 logically because it explains what happens in the video. It gives details about how the light behaves in the water.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 because it continues the explanation of what happens to the laser beam. It shows the effect of decreasing water flow on the laser.