Oh man, that's just the concrete trying to get a head start on the curing process without you! It's like the slab is so eager to help it decides to bake your thinset for you. Seriously though, on a hot day that slab is basically a giant heat sink that just drinks all the water right out of your mix. I've had to start so early in the morning it felt like a night shift, just to beat the sun. A good hose down right before you start is the only way to argue with it.

Ever notice how some folks act like a backpack is a paid passenger? I've found the best move is to stand right next to them and ask directly, "Is this seat taken?" while looking at the bag. It forces a clear answer. If they say no, you just wait silently for them to move it. The public pressure usually works. The worst I saw was a guy using a single, dry-cleaned shirt on a hanger to save a whole seat.

Yikes, that's why I always check my clicker against the old beam wrench first!