Are units that obvious ?

There are 2 types of physical quantities, the $\color{#D61F06}{\textbf{Fundamental}}$ and $\color{#D61F06}{\textbf{derived}}$ ones.

Fundamental quantities are those which do not depend upon other physical quantities, there are $\color{#D61F06}{\textbf{7 fundamental quantities}}$. They have own defined units.

$\color{#D61F06}{\textbf{Derived quantities}}$ are some function of the $\color{#D61F06}{\textbf{Fundamental quantities}}$, and they are infinite in number. Their units are depending on the units of $\color{#D61F06}{\textbf{Fundamental quantities}}$ in their dimensional analysis.

That's how quantities are defined, we all know this.

But do you know how $\color{#20A900}{\textbf{UNITS}}$ are defined ?

(Note, here I don't mean that $1 \text{ metre}$ is defined as $100 cm$)

$\color{#3D99F6}{\textbf{Here are the 7 fundamental quantities}}$ $\color{#3D99F6}{\textbf{with their units and definition of their unit.}}$

Their format- $\color{#69047E}{\textbf{Quantity}} \color{#20A900}{\textbf{---Unit}} \textbf{---Definition}$

$\mathbf{1.}\color{#69047E}{\textbf{Mass}}-\color{#20A900}{\textbf{kilogram}}$

The mass of a platinum-iridium cylinder kept in the National Bureau of Weights and Measurements, Paris

$\mathbf{2.}\color{#69047E}{\textbf{Length}}-\color{#20A900}{\textbf{metre}}$

The distance travelled by light in vacuum in $\dfrac{1}{299792458}^{th}$ part of a second or also defined as 1650763.73 times wavelength emitting from $Kr^{86}$

$\mathbf{3.}\color{#69047E}{\textbf{Time}}-\color{#20A900}{\textbf{second}}$

The time interval in which Cesium-133 atom vibrates $9192631770$ times.

$\mathbf{4.}\color{#69047E}{\textbf{Temperature}}-\color{#20A900}{\textbf{kelvin}}$

$\dfrac{1}{273.16}$ fraction of thermodynamic temperature of triple point of water.

$\mathbf{5.}\color{#69047E}{\textbf{Electric Current}} - \color{#20A900}{\textbf{ampere}}$

The amount of electric current that produces a force of $2\times 10^{-7} \textbf{N}$ per unit length, that acts between two parallel wires of infinite length and negligible cross-section area placed at 1m distance in vacuum.

$\mathbf{6.}\color{#69047E}{\textbf{Luminous Intensity}}-\color{#20A900}{\textbf{candela}}$

The amount of intensity on $\frac{1}{60000} m^2$ area of blackbody in the direction perpendicular to its surface at freezing point of platinum $2042$ K at pressure of $101325$ $N/m^2$

$\mathbf{7.}\color{#69047E}{\textbf{Quantity of substance}}-\color{#20A900}{\textbf{mole}}$

The amount of substance which has same number of elementary entities as in 12 gm of carbon-12.

Source- NCERT textbook. (Felt like sharing because this was new to me and may be new to many of us!)