Therein lies the insight: Even though you will continue moving forever--with each move taking you half the remaining distance to the wall--the total distance you travel can never be more than 2 feet, which is your starting distance from the wall. For mathematical purposes, the total distance you travel can be approximated as 2 feet, which turns out to be very handy for computation purposes. A mathematician would say that the sum of this infinite series 1 ft + 1/2 ft + 1/4 ft + 1/8 ft . . . converges to 2 feet, which is what our instructor was trying to teach us that day.