Where oh where did my lettuce garden go?

Let me preface this by saying that I am not a gardener. I have little experience and anything I do plant is lucky to survive. Having said that, here's my story:

Earlier this year I got a free packet of seeds in the mail from one of my freebie blogs I read. The packet was an assortment of organic lettuce seeds. So I thought I'd try my luck planting them in some pots. I had to transplant several times into larger containers as the plants got bigger. But then about a month ago I finally put them into the ground. I have a little plot of dirt outside my backdoor that I put them in. I ended up with about 10 various lettuce plants. And they were doing really well, despite the mild July we're having. I actually could have picked all the leaves off and made a nice salad. Of course I was going to give them some more time so that I could have more than one salad.

However, yesterday I go outside and notice something missing... My lettuce plants!!!

Some kind of bunny, deer, or other animal enjoyed a nice middle of the night salad. Ahhh! I know what you're thinking, she should have put a fence up. But I remind you that I am not a gardener and the seeds were free. You live and you learn right?

As to the culprit, I discovered only two curious clues. The first clue: This rock.

I found it sitting there next to my back step. I'm not sure where it came from but it is roughly the size of a brick. (I actually noticed this peculiarity before I saw the lettuce plants gone.) So I rolled it into the space between the dirt and my back step, where it fit so well I concluded that it probably originated from that spot in the first place. But when I woke up this morning it was out again!! And no, my kids are not moving the rock. Perhaps our hungry bandit has a hiding spot under my back step. Or maybe it's short and needs a stepping stone to reach my second clue:

A half eaten green tomato. This also happened twice. First, when I discovered the lettuce plants gone, and then again this morning.

My tomato plant is a whole other story altogether. For now I'll just say that it's right next to where the lettuce plants were and probably also needs a fence. (Not that I have much faith that it will ever get hot enough in Wisconsin for my 40+ tomatoes to actually turn red!)

So I guess you can say I do NOT have a green thumb. More like green thumb envy.