A noted sex therapist realized that people often lied about the frequency of their encounters, so he devised a test to tell for certain how often someone has had sex.
To prove his theory, he filled 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 until he came to the last man in line, an elderly gentleman, who was grinning from ear to ear.
"Twice a day," the therapist guessed. But the therapist was surprised when the man says no.
"Once a day, then?" Again the answer is no. "Twice a week?" "No." "Twice a month?" "No." The man finally said yes when the doctor got to "once a year."
The therapist is angry that his theory isn't working, and asks the elderly gentleman, "What the heck are you so happy about?"
The gent answered, "Tonight's the night!"