The spiky mystery seeds from last week were from a papaya fruit, as Rebecca guessed.
It has numerous common names, including pawpaw. The scientific name is Carica papaya.
It is a fast growing tropical plant, In the United States it is found in Florida and Hawaii, although it will grow in Southern California and Arizona with extra care to protect it against frost. The plant is extremely frost sensitive.
This lovely drawing is from Franz Eugen Köhler in Köhler's Medizinal-Pflanzen, downloaded from Wikipedia. It shows the male and female flowers.
Desert Tropicals has information on how to start the seeds and how to grow papaya in Arizona.
We found our papaya in the grocery store. We're going to try growing the seeds and we'll let you know how it turns out.
Have you eaten a papaya? Do they grow where you live?