Corrected sentence: The heart relaxes to fill with blood, then squeezes to push the blood out into the blood vessels.
Explanation for a 6-year-old:
- The heart is like a pump.
- First it relaxes (fills up with blood).
- Then it squeezes (pushes the blood out).
Small grammar notes: You don’t need to write HEART in all capital letters — just write heart. Adding the word "then" (or a comma before it) makes the sentence flow more clearly.