The Mystery Plant

We have a mystery watermelon plant growing in our pool yard. No one planted any seeds....and yet it's grown a few fruits now! The other evening I noticed a few creepers had made their way into the pool so I went out and dragged them back into a pile. And I noticed a rather larg-ish watermelon! BTW - up to this point, I had no idea what kind of plant it was....just that I knew it wasn't a weed and was pretty sure it was some sort of squash plant. Anyways, I picked the watermelon to determine if it really was a watermelon. So I brought it in, cut it open, smelled it, licked it, and declared it a watermelon. Although it was not ripe. This is what threw me off to begin with cause it was white inside. And I don't like watermelon so I never knew unripe watermelons are white inside. (I thought it was a weird giant cucumber)When I told this to David, he just looked at me. Thanks for making me feel like a moron! lol