Conway's Game of Life has two rules: a dead cell with exactly three live neighbours is born, and a live cell with two or three live neighbours survives. John Scholes, who had been drawn to APL in 1971 by the rumour that it could do Life in one line, finally filmed himself writing that line in 2009.
Read right to left. The board ⍵ is enclosed, rotated by minus one, zero and one in both directions to produce all nine shifted copies, which are summed to count neighbours. 3 4= tests each count against three and four, 1 ⍵∨.∧ folds in the survival rule, and ↑ unwraps the result. There is no loop, no index, no cell. The whole board is one value and the rules are applied to it at once. For many programmers this line was the first proof that a language could think in a different shape.