Mystery solved

A self-seeded vine started spreading across the yard recently

I thought it was pumpkin but was told the flowers were the wrong colour

Cucumber was my partner's first guess, but there didn't seem to be any seeds.

So, thinking it was zucchini, I added it to a couple of sauces and it seemed kinda sweet with pasta and tortillas.

Then I realised yesterday, it's a watermelon.

Seems so obvious now it's grown bigger.

Thinking I might try pickling it, since it's too late in the season for the fruit to mature.