Mushrooms are like little sponges! Rinsing them in water, or worse, soaking them in a bowl of water, just gives them a chance to drink up excess water like a dry dish sponge would. Because mushrooms already have a lot of water in their makeup, you typically give the moisture time to cook off, or add less liquid to a recipe if it's being baked with mushrooms. Rinsing them in water simply adds more, which in most cases we don't want!
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