A quick spray in the lens of each goggle before a training swim, rub the fluid around briefly with a finger and rinse in the pool water before putting then on. Works a treat for me, no fog build up in my goggles no matter what kind of swim session I am doing - pool or open water. In short this works really well. Also, the bottle lasts for absolutely ages....a good six months when swimming three times a week. I would certainly recommend this to anyone who does a reasonable amount of swimming.