Why I can’t see the green box? (Parent/child)

Question

https://code.sololearn.com/Wt20gl716Ubv/?ref=app

Answer 1

positioning

https://code.sololearn.com/W4dGFJx8wFW7/?ref=app

Answer 2

if i am changing the height to 300px
the green box gets under the red..
why not on top?

Answer 3

so to get the boxes on top of each other i need the parent to be absolute?
i try that mess up the rest of the boxes