Look straight down into a village well and you see exactly this: a square wooden frame of crossed beams holding the walls back from the water below. The oracle-bone scribes drew that frame from above as a simple crossed grid, and the shape has barely moved in three thousand years — the modern 井 is still four strokes making the same crossing pattern.
A piece of culture
The well-field system Because the grid of 井 looks exactly like a tic-tac-toe board, Zhou-dynasty thinkers used it to picture an idealized way to divide farmland: the 井田 ("well-field") system split a square into nine equal plots in a 井 pattern, gave eight of them to eight families to farm for themselves, and had all eight jointly work the shared center plot as tax for their lord. Mencius described it centuries later as a model of fairness, whether or not it was ever practiced quite so neatly — and the character still marks everyday gathering places today: 市井 ("market and well") means the bustling, sociable streets of ordinary town life.
Use it in a real word
天井
tiānjǐng · tin1 zeng2
skywell — a small roofless courtyard at the center of a traditional house that lets in light, rain and air; you already know 天