Besides being official weight and size, the Night Flyer Constant On Golf Ball is a very sophisticated piece of equipment designed to light-up upon sensing a strong impact. Once activated, the built-in computerized timer will keep the ball illuminated for 8-10 minutes after which the switch will automatically turn off the light to preserve battery life unless…the ball receives another firm impact. Anytime the computer senses a strong impact the timer will be reset to zero and restart the 8-10 minute light-up period. Batteries in each ball are sealed and are NOT replaceable. Night Flyer Golf Balls are protected by Patent #6,042,487. Sold by the individual ball. Please order in increments of 1 golf ball.