In 1200 A.D., a group of nomadic hunters called the Aztecs migrated into Mesoamerica in search of food and wild game. By 1325 A.D., the Aztecs had settled in present day Mexico City, and had built a large, beautiful and powerful city on a small island which they called Tenochtitlan.
As the population of Tenochtitlan grew, the island became too small. Their innovative solution was to make the island bigger. They built large wooden rafts, which they covered with mud and secured to the lakebed with stakes. Many people lived and farmed on these floating parcels of land.
The pleasure we derive from doing favors is partly in the feeling it gives us that we are not altogether worthless.