A man brings two drinking glasses and sets them on the bed-side stand. One is full of water and one is empty. His wife asks him what the glass of water is for. "In case I get thirsty and want a drink."
"And what's the empty glass for?"
"In case I don't get thirsty."
The high incidence of shower deaths of computer programmers was finally explained today when an ivestigator noticed the instructions on the shampoo bottle: "lather, rinse, repeat".