There are n students in a school. Some of them are friends with each other. You want to make the school clean. So you have to give ice-cream to each student so he or she starts planting trees. When one plants a tree, he/she tells it to all of his friends and they start to plant trees and tell their friends too (without any ice creams) and so on.
This is a companion discussion topic for the original entry at https://toph.co/p/plant-tree-save-life