Correct sentence:
A porcupine's long, sharp quills keep it safe. They are also hollow and can help it swim.
Step-by-step explanation (easy words):
- Start with a big letter: "A" instead of "a".
- Say "A porcupine's" to show the quills belong to one porcupine.
- Put a comma between two describing words: "long, sharp."
- Use "hollow" instead of "filled with air," and add "can" to show it sometimes helps the porcupine swim.
Nice work! Short and clear sentences are easier to read.