Tra La! The Magical Mystery Gourd!

So a few weeks ago we noticed a strange plant growing in our back yard. Since we were growing pumpkins on the side of the house, and it had a similar appearance we assumed it was some kind of squash. As the mystery plant grew larger and larger our pumpkin started dying off. After some extensive internetting I discovered that the pumpkins had become the feast of the measly, grubbing squash borers. However the backyard plant seemed unaffected. It thrived, despite the fact that I never once watered it. Big yellow blossoms started bursting out all over, and I was convinced we were dealing with a rogue pumpkin. The leaves were a slightly different shape, but there are lots of different varieties of pumpkins, right? 

Well, over the weekend we discovered, with the help of some friends, a large fruit growing on the vine. That's when the whole debate sparked. One friend is insisting it is a pumpkin while another is insisting it is a watermelon. I checked in with the world wide webulator (tm SOB) but found no evidence that convinced me that it was either. 

That's where you come in dear internets! Help a novice gardener out, will you? Observe the photos below and weigh in on what you believe to be the identity of the Magical Mystery Gourd. Once whatever it is is ripe we'll hack into it and reveal the truth. 

I have another gardening type question, but I'll leave it for another day. I've had enough for one day...