A mechanically expandable foam sheet has comprising a plurality of discrete slits therein that allow the foam sheet to be expanded from an unexpanded foam sheet configuration to an expanded foam sheet configuration. The expandable foam sheet has a density of from 14 g/liter to 48 g/liter in the unexpanded configuration and a thickness of from 6 mm to 50 mm. The slits are of a kind and of a size and arrangement that the expanded foam sheet locks into the expanded configuration by exhibiting a pressure-to-close of at least 5 gm/cm2. Each of the slits provides only a single free volume in the expanded foam sheet. An expandable foam sheet having serpentine-shaped expansion and locking arms is also disclosed, as are various cushioning articles comprising expanded foam sheet.