White to move and checkmate in exactly five moves, Sam Loyd's 1861 trap
Sam Loyd composed this chess problem to win a bet: a friend claimed he could always guess which piece delivers checkmate, and picked the lowly pawn on b2 as the least likely candidate. White must force mate in exactly five moves. Which piece actually delivers it?
Reveal the answer
That very pawn on b2, it survives a gauntlet down the board and promotes on move five to deliver checkmate (1.b4! Rc5+ 2.bxc5 a2 3.c6 Bc7 4.cxb7 any 5.bxa8=Q/B#). First published in the London Era in 1861, it's the original example of what composers now call the 'Excelsior' theme: a single pawn marching the length of the board to seal the mate.
— Sam Loyd, Excelsior (chess problem) — Published in the London Era, 13 January 1861