Reduction from P to NP
Note: this answer assumes that we are considering polynomial-time reductions (and not e.g. log-space ones).



If $A$ belongs to $P$, then we can reduce $A$ to any nontrivial language, i.e. any $B\neq \emptyset,\Sigma^*$.

Reducing $A$ to $B$ can be done as follows. Fix $b_1\in B, b_0 \in \overline B$ arbitrarily. The reduction algorithm then takes an input $x$ and tests whether $x\in A$: if that holds, return $b_1$, otherwise return $b_0$.

Note that all of this can be run in polynomial time since $A$ belongs to $P$. This allows to solve the $A$ instance in the reduction algorithm itself, trivializing the reduction.

This proves that problems in $P$ are indeed the easiest possible ones, according to the "polynomial-time reduction" preorder.