What's the Name of That Book??? discussion

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Hello? Is Anybody There?
SOLVED: Children's/YA
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SOLVED. Children's Fiction: Alien is found in child's garden; both children compare lives/gravity/and reflect on philosophy of existence. [s]
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Spoilers below.
Read this in 2000-2001 academic year but was property of my teacher.
Book was very philosophical and had illustrations - including one of the child (main character)'s mother breastfeeding his baby sibling.
A child finds an alien in the garden, possibly because the alien's spaceship is not working.
The alien is short and looks like a child but in fact is much older. They compare how long a year is on the alien's planet to Earth (much longer) and how old they would be on the planet.
The gravity is much heavier on the alien's planet so they are able to jump very high.
The main character child is fascinated by the fact that the alien doesn't have a belly button so the two of them compare how the two species are born. This is when the human child shows photos of his mother breastfeeding. People used to turn to this page to show others because of the drawing of breasts.
There were lots of "Big Questions" and the book ends with something along the lines of "If there isn't a god, how did the universe come to be. If there is a god, what is the universe?"
The child may have named the alien / or the alien's name might have been Nod / Ned something like that?
Both learn a lot from each other and the alien is eventually picked up by their people.