A "hand" is a unit of measurement commonly used in the context of measuring the height of horses. In the United States and some other English-speaking countries, a hand is defined as exactly 4 inches or 10.16 centimeters.
Horses are typically measured in hands from the ground to the highest point of their withers (the ridge between the shoulder blades). This measurement is a convenient way to describe a horse's height without relying on feet and inches or meters.
For example, a horse that measures 15.2 hands is 15 hands and 2 inches tall. Horses can vary in height, and the hand system provides a standardized way to communicate a horse's size, which is particularly important in horse racing, equestrian sports, and the horse industry in general.
A "fermi" (symbol: fm) is a unit of length used in physics to describe extremely small distances at the atomic and subatomic scale. One fermi is equal to 10^-15 meters or 0.000000000000001 meters. It is named after the Italian physicist Enrico Fermi, who made significant contributions to nuclear physics and particle physics.
The fermi is particularly useful for describing the sizes of atomic nuclei and the distances between particles within atomic nuclei. For example, the typical diameter of an atomic nucleus is on the order of a few femtometers (1 femtometer = 1 fm). It is also used in high-energy particle physics to describe the cross-sectional area of particle interactions.
In summary, the fermi is an essential unit of length for understanding the microscopic world of atoms, nuclei, and subatomic particles, where distances are incredibly small.