Fermat test detects compositeness

Compute. $2^6=64\equiv1\pmod{21}$ (since $64-21\cdot3=1$). So $2^{20}=2^{18}\cdot2^2=(2^6)^3\cdot4\equiv4\pmod{21}\neq1$.

Conclusion. $2^{20}\not\equiv1\pmod{21}$, so $21$ is composite.

Remark. This is the Fermat primality test: if $a^{n-1}\not\equiv1\pmod n$ for some $a$ coprime to $n$, then $n$ is definitely composite.