Teach your child a lesson in assumptions. Give her two options: She can have $500 now, or she can have one penny now plus double the amount every day for the next 30 days. If she takes the $500, show her how she missed out on $10.7 million!  It's solvable by multiplying 1 x 230 where 1 is the penny, 2 is the doubling factor, and the 30 accounts for each day.