A mole of lightning

Lightning strikes Earth about 100 times each second, or 8.6 million times a day. Based on the information above, estimate the minimum number of years it will take for one mole of lightning to hit the earth.

Answer:  1.9 x 1014 years (Did you remember to take into account leap years? Ask your students if leap years makes a difference in their answer. Then you can have a discussion on significant digits.)