The milligram is a unit of mass (acceptable for use as weight on Earth) and is a submultiple of an SI base unit with the symbol mg.
1 milligram is equal to 0.001 g or 1/1000 g (one thousandth of a gram).
There exists 2 submultiples between mg and g - namely the decigram and the centigram but these are not commonly used in everyday life. The milligram is commonly used when expressing parts of a gram; especially in medicine applications.
A mosquito weighs about 2.5 mg, as does a grain of sand.
The stone is a unit of mass (acceptable for use as weight on Earth) and is part of the imperial system of units. It has the symbol st.
In historical times actual stones were used as weights, a practice that was adopted worldwide for more than 2000 years.
In Europe, up until the 19th century, the stone was frequently used in the trade industry to measure weight. Each country had their own weight of the stone which varied between regions and what was being traded. As a result, the metric system was adopted in most European countries.
In England in 1389 a stone of wool was characterized as weighing fourteen pounds (lbs). Despite the fact that a stone of different materials would not necessarily weigh exactly fourteen pounds, the stone became accepted as weighing exactly 14 lbs.
The stone is also used in sports. In horse racing it is used to describe the weight that a horse has to carry. The weight includes the jockey as well as overweight, penalties and allowances. It is also used to express human bodyweight in sports such as boxing and wrestling.
In the UK and Ireland people will often use stone and pounds (e.g. 11 st 5 lbs) to express their weight. In contrast people in the United States will most commonly use just pounds (eg. 159 lbs).
Although the Stone has not been recognised in the UK as a unit of weight since 1985, it is still the most common and popular way of expressing human weight in this country.