Measurement is used to determine the size of things. Units are used to give a value to a measurement in a standard way.
Length measures distance in centimetres, metres, inches and feet.
Mass measures how heavy something is, and may be measured in grams, kilograms, ounces or tons.
Capacity measures volume, using litres, pints or gallons.
Note: Mass is not the same as weight. On the moon, you would weigh `frac(1)(6)`th of the weight as you would on earth, yet your mass is still the same. The difference between mass and weight is the effect of gravity.
1. What is wrong with this statement? "I want a piece of wood 25 millimetres across by 50 millimetres deep, and 6 feet long."
Answer: The first two measurements use Metric units, the third is an Imperial unit.
Imperial and Metric units are not normally mixed.
2. What units would I use when filling a car up with petrol?
Answer: Litres (metric) or gallons (imperial).
Litres and gallons are measures of capacity.