If these look familiar, it's because
we also had them in Michigan last fall, and in California the year before; they're black walnuts, and they make walking in the grass difficult, because they turn under your feet. When they're on the sidewalk or street, they leave gooey black marks....I guess they'd do that in the lawn too, but it's not as obvious as when cars are crushing them. They're about the size of a golf ball.