Mass is the amount of matter that an object contains and is measured in kilograms. The weight of an object is the force that is acting on the object due to gravity. Imagine an object with a mass of 1 kg. On Earth this object is pulled down by gravity and you feel and measure this force as weight. In space however there is no effect of gravity and the object would be weightless - but it would still have a mass of 1 kg. On the Moon, where the effect of gravity is 1/6th that on Earth, the same object would still have a mass of 1 kg but it would weigh 1/6th of what it weighs on Earth.
(Weight should technically not be measured in kilograms, The correct unit is the Newton, N, which is the force that the mass exerts due to gravity. 1 kg exerts a force of 9.8 Newtons.
So if your mass is 70 kg - you actually weigh 686 Newtons! This becomes more important when your child passes the 11 Plus, goes to grammar school and begins to study physics.)
This is a difficult concept to understand but it is why we should use the term mass and not weight when dealing with this type of measurement.
1 kilogram is the mass of a litre of water or a bag of sugar.
1 gram, g, = 1/1000 of a kilogram - about the same as a paperclip.
1 tonne = 1000 kg = 1000 litres of water or a small car.
In the UK we use a combination of Metric and Imperial units, which means that your child has to understand and be able to compare both types of measurement.
Imperial units of mass are stones, pounds and ounces.
One stone = 14 pounds - just over 6 kg.
One pound (lb) = 16 ounces (oz) = 454 grams (just under ½ kg).
One ounce = 28 grams - about three pound coins.
The kitchen is a good place to measure different items - modern electronic scales often allow you to switch between metric and imperial measures.