Bill, an experienced sky diver, was getting ready for a jump one day when he spotted another man outfitted to dive wearing dark glasses, carrying a white can and holding a seeing-eye dog by a leash. Shocked that the blind man was also going to jump, Bill struck up a conversation, expressing his admiration for the man's courage. Then, curious, he asked, "How do you know when the ground is getting close?"
"Easy," replied the blind man. "The leash goes slack."