Extemporaneous lesson on exponential growth

My sharpest student was badgering me about paying his $44 phone bill. I gave him a penny. "Wow" he said. I then gave him another; "Now I've doubled it. Come back later and I'll double it again." He said that it would take for ever that way. BAM! Teachable moment: below is what I wrote on the board to explain what would happen.

He was intrigued. :) (It also involved log, which was painful.)