Button mushrooms are sponges: you do not soak them, you wipe them. While cooking, they release water, so you cook them on high heat in a pan that is not too full, and you salt at the end.
Firm, with a closed, dry cap, no dark spots.
Cut the earthy end of the stem. Wipe the caps with damp paper towel, or rinse them very quickly under water and dry them right away.
Two minutes for a punnet.
In the fridge, in a paper bag or a container left ajar, four to five days. Never in a closed plastic bag, they go mouldy.
The mushroom risotto: a punnet of button mushrooms in slices.