Solitaire

Klondike, the solitaire that came with every computer anyone has ever owned. Seven piles, four foundations, and the whole deck to unpick — build the suits up from ace to king and strip the table bare. Not every deal is winnable, which is part of the deal rather than a bug: roughly a fifth cannot be solved however well you play them, so knowing when to start again is a real skill. The useful habit is emptying a column early. An empty column takes a king, and a king is what unblocks everything underneath.