Scrub them well. Place an oyster, rounded side down, on the work surface. Grip the oyster with a towel to protect your hand, leaving the hinged end exposed, and force the tip of an oyster knife into the hinge. Twist the blade to pry the shells apart. Slide the knife along the inside of the upper shell, cutting the oyster from the shell, but leave it in the shell. Set the shell on a bed of rock salt or crushed ice. Repeat with remaining oysters.