This is a common weed that I've heard about for a long time, but I never actually IDed it before.
I went to a local field/dog park in search of some of the more ephemeral spring wildflowers. While searching, I found this tiny little star shaped flower. And I mean tiny! Look at it against my thumb:
It took me a while to ID this as Chickweed because of the deep cleft in the petals. Count them. How many do you see? The answer is 5, but for the longest time I kept counting 10 and using that characteristic to ID. But the Stellaria genus is in the family Caryophyllaceae which can have very clefted (or "pinked") petals.
It turns out this is also an edible plant, the greens being good blanched or raw and added into salads or steeped into a tea. However as I found this patch in a dog park, I don't think I'll be eating them no matter how well I wash them first.