International System of Units explained to kids

The International System of Units, or SI, is the most widely used system of measurement. It is used in almost all countries around the world. There are seven base units in the SI system: the second, meter, kilogram, ampere, kelvin, mole, and candela. The SI system also has two supplementary units: the radian and the steradian. The SI unit of time is the second. It is the standard unit of time in the International System of Units. The second is defined as the time it takes for a Cesium-133 atom to vibrate 8,192 times. This is a very precise definition of the second. The SI unit of length is the meter. The meter is the length of the path traveled by light in a vacuum in 1/299,792,458 of a second. This is the speed of light in a vacuum. The SI unit of mass is the kilogram. The kilogram is the mass of a platinum-iridium cylinder kept at the International Bureau of Weights and Measures. The SI unit of electric current is the ampere. The ampere is the current that flows through two infinitely long parallel wires that are 1 meter apart in a vacuum. The SI unit of thermodynamic temperature is the kelvin. The kelvin is the 1/273.16 of the thermodynamic temperature of the triple point of water. The SI unit of amount of substance is the mole. The mole is the amount of a substance that contains as many elementary entities as there are atoms in 0.012 kilograms of carbon-12. The SI unit of luminous intensity is the candela. The candela is the luminous intensity of a source that emits monochromatic radiation of frequency 540 x 10^12 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian. The SI system also has two supplementary units: the radian and the steradian. The radian is the SI unit of angle. It is the angle between two lines that intersect at the center of a circle. The steradian is the SI unit of solid angle. It is the angle between two lines that intersect at the center of a sphere.