First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic about penguins and their unique method of mobility. It does not refer to any previous information and sets the stage for what follows. This follows the rule of Topic Sentence. Why other lines are not chosen? Paragraph 2 starts with 'These bubbles', which refers back to something not yet introduced. Paragraph 3 starts with 'While swimming', which suggests a context that is not yet established. Paragraph 4 starts with 'Engineers', which assumes prior knowledge of the topic. Paragraph 5 starts with 'The water pressure', which also assumes prior context.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the discussion by showing how the idea from penguins is used in engineering. It summarizes the main points and connects back to the overall theme.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects to Paragraph 1 because it continues the topic of penguins and their method of mobility. This follows the Logical Order rule as it explains how penguins swim.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 logically follows Paragraph 2 because it explains what happens to the air trapped by the feathers. This follows the Cause and Effect Relationship rule.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects to Paragraph 3 because it describes the function of the bubbles created by the trapped air. This follows the Broader to Specific Strategy rule.
• Pair 4 (Paragraph 4 → Paragraph 5): Paragraph 5 follows Paragraph 4 because it discusses how engineers use the idea of bubbles to improve technology. This follows the Topic Example rule.