A group of boys are lined up on one wall of a dance hall, and an equal number of girls are lined up on the opposite wall. Both groups are instructed to advance toward each other by one quarter the distance separating them every ten seconds. When do they meet at the center of the dance hall? A mathematician says they will never actually meet because the series is infinite. A physicist says they will meet when time equals infinity. An engineer says that within one minute they will be close enough for all practical purposes.