Depends on the age. Simply ... "Once upon a time ..."

There was a neighborhood of houses. In one house, the family went crazy and became bullies. They started moving their fences into the yards of their next door neighbors. Everybody in the neighborhood saw what they were doing, but did nothing and hoped they would just stop. One day the crazy people broke into the neighbors houses and took them for themselves, one after the other - some neighbors up the street tried to help, but were also taken over or barely kept the bullies out. The neighbors down the street teamed up with the bullies for little while, but then the bullies tried to take over their house.

Eventually help from outside the street arrived and pushed the bullies back into their own house, and took away their clubs. Everybody's house were wrecked or burned down, and as the neighbors up the street rebuilt their houses they saw that one big neighbor down the street never left the house they helped take back from the bullies, and were acting the same way as the first bullies. So the neighbors up the street realized they were weak by themselves, but together were stronger than the new bullies, and agreed if one them were bullied all of up the street neighbors would come to help all at once at the same time. Some in the neighborhood didn't join at first, but eventually did, and some never did, but there is lot less bullying in the neighborhood.