A noted sex therapist realized that people often lie about the frequency of their encounters, so he devised a test to tell for certain how often someone had sex.

To prove his theory, he filled up an auditorium with people, and went down the line asking each person to smile. Using the size of the person's smile, the therapist was able to guess accurately how often each person had sex. The last man in line was grinning from ear to ear.

"Twice a day," the therapist guessed, but was surprised when the man said no. "Once a day, then?" Again the answer was no. "Twice a week?"

"No."

"Twice a month?"

"No."

When the doctor asked, "Once a year?" the man finally said yes.

The therapist was angry that his theory hadn’t worked with this individual, and he asked the man, "What the heck are you so happy about?"

The man answered, "Tonight's the night!"