40 boxes or 39 boxes joke

The math professor just accepted a new position at a university in another city and has to move.

He and his wife pack all their belongings into cardboard boxes and have them shipped off to their new home. To sort out some family matters, the wife stays behind for a few more days while her husband has already left for their new residence.
The boxes arrive when the wife still hasn't rejoined her husband. When they talk on the phone in the evening, she asks him to count the boxes, just to make sure the movers didn't loose any of them.

"Thirty nine boxes altogether", says the prof on the phone.

"That can't be", the wife exclaims. "The movers picked up forty boxes at our old place."

The prof counts once again, but again his count only reaches 39.

The next morning, the wife calls the moving company and complains. The company promises to check; a few hours later, someone calls back and reports that all forty boxes did arrive.

In the evening, when the prof and his wife are on the phone again, she asks: "I don't understand it. When you count, you get 39, and when they do, they get 40. That's more than strange..."

"Well", the prof says. "This is a cordless phone, so you can stay on the line and count with me: zero, one, two, three,..."