Tidal Flats

Oracle Text

UU: For each attacking creature without flying, its controller may pay 1. If that player doesn't, creatures you control blocking that creature gain first strike until end of turn.

Card Rulings

10/4/2004 Can be used more than once in a turn. Each time, the opponent can pay to prevent giving first strike to a creature, but this payment only stops the current activation and not future activations of Tidal Flats.
10/4/2004 The payment is made when the Tidal Flats effect resolves.
10/4/2004 First Strike is granted to your creatures currently blocking non-Flyers when this effect resolves.