How it works
Mia and Leo each got 16 pretend coins and used the same garden rule: one new coin for every two coins in it.
Mia planted at once. Leo kept his in a pouch for two years first.
After four years Mia had 81 coins and Leo had 36. Nobody added any more coins.
The only difference was when they started. Starting sooner gave Mia's coins two more years to grow.