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 it right to left. The board ⍵ is enclosed, then 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 and no index. The whole board is one value and the rules apply to all of it at once. Scholes' video became the best known demonstration of APL, and Conor Hoekstra remade it with added explanation in 2021.