Here are your corrected sentences:
- The iris is the coloured part of the eye.
- The pupil gets bigger in the dark and smaller in the light.
- The sclera wraps around the eyeball and protects it from injury.
What I changed and why:
- I only changed one small thing: "an eyeball" → "the eyeball" so the sentence matches the earlier use of "the eye" and sounds more natural.
- All sentences already had correct capitalization and punctuation (they start with a capital letter and end with a period).
- Spelling note: "coloured" is British English. In American English it is spelled "colored." Either is correct depending on which variant you use.
Simple explanations for a 6-year-old:
- Iris — the coloured part of the eye (like brown, blue, or green).
- Pupil — the black circle in the middle that gets bigger in the dark and smaller in bright light.
- Sclera — the white part that wraps around the eyeball and helps protect it.
Nice work — your sentences were already clear and mostly correct!