Cultivated mushrooms, like button mushrooms and portobello, can be cleaned using a dry cloth or paper towel to wipe off any excess 'dirt' they may have on them. Once wet, it is tough for mushrooms to dry completely, which makes it less likely they'll get those crispy edges when you sauté or bake them. Mushrooms also act as a sponge; you don't want them to absorb excess water before seasoning.