Mass vs. Weight, and Why Stones Are Still a Thing
Colloquially "weight" and "mass" get used interchangeably, but they're technically distinct: mass (kilograms, pounds as a mass unit) is an intrinsic property of an object, while weight is the force gravity exerts on that mass, which technically varies with gravitational strength (an object weighs less on the Moon despite having identical mass). For everyday conversion purposes on Earth this distinction rarely matters practically, but it's the reason "pounds" can refer to either a mass or a force unit depending on context, a genuine source of physics-versus-everyday-usage confusion.
The stone (14 pounds) remains in common everyday use specifically in the UK and Ireland for body weight, even though it's fallen out of use for nearly everything else — a specifically regional survival of an otherwise largely retired imperial unit, which is exactly why converting a stone-based body weight to kilograms or pounds is still a genuinely common practical need, not a purely historical curiosity.
Metric tons (1,000 kg) and US "short tons" (2,000 pounds) are yet another source of ambiguity worth flagging specifically — the two aren't the same weight at all (a metric ton is about 10% heavier than a US short ton), and "ton" alone, like "gallon," is genuinely ambiguous without specifying which convention is meant.