The unit of volume is the metric litre which is used for water, petrol and other liquid. A litre is equal to a 10 cm by 10 cm by 10 cm (or cubic decimetre. There are approximately 3.78541 litres in one US gallon. A millilitre is a 1cm by 1cm cube - this is also called ml or cubic centimetre or one CC. [Cook]]s often use this measurement. A typical teaspoon is about 5 ml in size.
The unit of mass is the metric gram which; on Earth is quite light. On Earth it weighs the same as a 1 millilitre of water at exactly 0 degrees Celsius, a bit too warm to turn into ice. A more common measure is the kilogram. This weighs the same as a litre of water - exactly 1000 grams because 10 x 10 x 10 = 1000. The metric tonne is 1000 kilograms, a million grams.
The unit of time is the second. The minute (60 seconds) and hour (60 minutes or 3600 seconds) are larger units. The day is usually said to be 24 hours, but is actually a tiny bit longer. This difference is corrected at the end of every year. A week (7 days) and month are also standards in most places, but there are different calendars. These are not part of metric but are used in finance and other industries that set some standards.
Standards are very important. Each unit is a set size. A distance or length or volume or mass or span of time being measured is described as a certain number of these units. Sometimes more than one unit is used to get better accuracy, or a smaller unit is used - so one can say "one metre, 95 centimetres" or just "195 centimetres".
The imperial units now used mainly in the United States: inch, foot, yard, mile, and the US gallon, smaller than the old imperial gallon (UK Gallon), about 4.5 litres, which is now not used anywhere much. Some older bottles, jugs and liquid containers are this size. There are 128 fluid ounces in a US gallon, but there are also 16 ounces in a pound, an older measure of weight that is 453.6 grams. These are two different measurements with exactly the same name - it is only when measuring water that they actually mean the same.
Science, medicine and engineering use larger and smaller units of measurement than these, and talk about them more exactly. For instance the difference between mass and weight matters a lot more in these fields.
