What is this? I have reason to believe that it's some sort of Lactuca, but I'm wondering, if it is indeed Lactuca, what species it is.
Aside from the picture, all I really know is that it releases white sap when cut or broken.
Milk thistle, Sonchus sp?
Most thistle have beautiful flowers, but are considered weeds (especially in crop fields). Don't let the seeds spread if you don't want 1000 little/big babies coming up next year.
If I recall from last year, these end up having some kind of flowers on them later in the year.
Are there small prickles along the backs of the leaves? Although the lower leaves on your plant look very much like Sow Thistle, Sonchus, the upper leaves look more like a Lactuca. Both would have milky sap. Can you post again when they flower?