We'll make quick work of this one (while trying not to sound like mom): you can't just eat mushrooms out of the box. They're dirty. Only this time, they're literally dirty, like with actual dirt, and can't just be "washed." Here's how you clean them.
First, trim or remove the stems. When that's done, don't just toss them in a colander and run water over them, it won't wash the dirt off and will waterlog the mushroom's gills, and nobody likes a soggy mushroom. Instead, quickly run a paper towel under the faucet, wring out the excess water and gently, tenderly wipe the dirt from the mushroom's surface and underside, because nobody likes a crunchy mushroom (unless it's been washed and THEN battered and fried). Don't apply too much pressure while doing this, or you'll rub off the delicate top layer.
More Whatchamacallit on Food Republic: