What the heck is that?

My prize watermelon from the garden is apparently some kind of pumpkin-watermelon hybrid freaky fruit. I was so excited to see it growing. I took pictures of it and was positively giddy when we harvested it. I almost set up a show and tell in the driveway just so neighbors could admire the fruit.

Working late on Tuesday, I get this call from John.
J: Hey........ yeah, um, the watermelon... well, I cut it open.
L: Is it sweet? I am so excited.
J: It's a pumpkin.
L: WHAT?
J: It's a pumpkin.
L: It can't be. It's green. It's shaped like a watermelon. It's supposed to be a watermelon!
J: Well, it's a pumpkin.