Ready to find out what the mystery seed of week 7 was?
It was the seed of the northern catalpa, Catalpa speciosa.
These trees have big, heart-shaped leaves.
They are often grown for their big showy flowers.
Here's a better look:
I believe there are a few southern catalpas (Catalpa bignonioides) in Arizona, but the northern catalpas from the photographs above are from upstate New York and Pennsylvania.
We have a tree that belongs to the same family growing in our yard, but that will be for another time.