On a fast green, most golfers only account for slope. They miss the second variable entirely: grain.

Grass grows toward the setting sun, generally, which means late-afternoon putts often break differently than the same line would've broken that morning. A shiny, silvery look to the green means you're looking down-grain — the putt will run faster than it looks. A duller look means into the grain, and it'll die quicker than expected.

It's a small thing most weekend golfers never learn to notice — and it's the difference between a putt that trickles past and one that finds the edge.