Speed, mass, hot and cold

Mass is a quantity showing how “heavy” a body is. In daily life we say “weight”, but in science what we find when we weigh apples in a shop is their mass. Mass is measured with scales and expressed in grams (g), kilograms (kg) and tonnes (t): 1 kg = 1000 g, 1 t = 1000 kg. Grams and kilograms suit an apple or bread, tonnes suit a lorry. Whether an object floats or sinks in water depends on what it is made of. Cork, polystyrene and wood float because they are lighter than water; a steel cube, a stone or a nail sinks because it is denser. Objects of the same size can have different masses. A big steel ship floats because it is hollow and full of air.