Straight four = alive
Four empty points in a row form the straight four, the simplest unconditionally alive shape: it holds two vital-looking centre points, and the attacker can never take both. Whichever one White throws a stone at, Black answers on the other, and the space falls into two eyes — the end points are half-finished eyes on their own. Try to break it anyway: the attempt itself teaches why no probe works. White to play: attack the shape and watch Black's answer.