The proprietor of a successful optical shop was instructing his son on how to charge a customer.

"After you have fitted the customer's glasses," he said, "and he asks you what the charge will be, you say, '$10.' Then see if he winces."

"If the customer doesn't wince you say, 'For the frames. The lenses will be another $10.'"

"If he still doesn't wince you say firmly, 'Each.'"