Each other player trashes a card from his hand.
He gains a Copper per Coin in the cost, putting them into his hand.
Each player chooses which card he or she will trash. The Coppers are gained into the player's hand, so if Cannon is played multiple times in a turn, players can trash the Coppers they have gained from a previous attack in response to a new attack.
Potion symbols are ignored, only he Coin values are counted.
A player's hand can exceed the normal size because of this. For example, if a player chose to trash Possession they would gain 6 Coppers into their hand.
Reactions which affect gaining cards such as Watchtower or Trader can be used in response to the gain.
Trashing a card with zero cost, such as a Copper or cards with costs reduced to zero by the Bridge wouldn't cause the gain of any cards.
If the Supply of Coppers runs out players still trash a card and gain nothing (or as many Coppers as they can if there are fewer than the cost of the card).