TOTAL VIDEO on Instagram: "The post features a 15-second video demonstrating the fingernail test on mirrors: no gap between finger and reflection indicates a standard mirror, while a gap suggests two-way glass, framed as a surprising safety tip shared by a friend during travel. This method identifies second-surface mirrors common in two-way setups but lacks perfect accuracy, as minor gaps can occur in regular mirrors due to manufacturing; peer-reviewed optics sources confirm it relies on reflect