Corrected sentences:
A porcupine's long, sharp quills keep it safe. The quills are also hollow, which helps the porcupine swim.
Why I changed them (easy steps):
- I added the word "A" at the start so the first sentence begins properly and sounds complete.
- I wrote "The quills are also hollow" to make the second sentence clear about what is hollow.
- I added "which" and a comma to connect the idea that being hollow helps the porcupine swim.
- I used "the porcupine" instead of only "it" to make the meaning very clear for a young reader.