Let $k$ be a field with zero element $0_k$ and multiplicative identity $1_k$. Let $\mathcal{S}$ be the set of all subfields of $k$, and let $P:=\bigcap_{F\in\mathcal{S}}F$ be the prime subfield of $k$. If $\operatorname{char}(k)=0$, then $P \cong \mathbb{Q}$. If $\operatorname{char}(k)=p$ for a prime number $p$, then $P \cong \mathbb{F}_p$.