Mushroom identification is tricky. (Oh before I go any further, did you notice these look kind of like dicks? It was pointed out to me on a couple mushroom identification sites, by what I can only assume were professional comedians.) Mushrooms change over time, starting as small knots of material that become spherical, then perhaps ovoid, before unveiling themselves and producing spores.
This group of mushrooms will absorb moisture, which will cause them to expand. The stalks will push the caps skyward, and the caps will spread wide, the smooth brown surface breaking into dozens of reddish brown scales on a white field. The underside of the cap, and the stem, are also white, though they will develop a reddish stain with age or bruising.
Helpful experts pointed me toward Leucoagaricus americanus*, for this fungus. The habitat is fairly distinctive: wood chips and tree removal sites. This was in a mulched area in a parking lot.