When you bake or cook, you can run into some measuring problems. Europeans like to use the metric system and give set weight or fluids in milliliters, while Americans and British recipes rely on cups and gallons. A cup is actually a set measurement, which equals 250 milliliters or a quarter of a liter.
Historically most measurements of length involved the human body. Makes sense, doesn’t it? It was the one thing you always have at hand, so to speak. For instance, an inch was the width of a mans thumb. This resulted in totally varying lengths. In an attempt to pin down a more exact measurement, King Edward II of England ruled that an inch was not measured by a body part but by barley. When you put three grains of barley end to end, lengthwise, you had an inch. Not exactly perfect or scientific, but the deviation was way smaller.
The metric system was literally invented from scratch. At the height of the Age of Enlightenment, French developed the metric system. The basic requirements were a decimal notation and general accessibility for all. The basic units were taken from the natural world. The meter was originally one tenth-million of the distance from the equator to the north pole (however they figured that one out), today it is based on the wavelengths of krypton. Measures of volume were derived from cubing measures of length. The first official introduction of the metric system was during the French Revolution in 1799. Today the United States, Myanmar and Liberia are the only countries which have not officially adopted the metric system.