Can $\emptyset$ be reducible to any other language?
$\emptyset$ can be reduced to any other language $L$ except&nbsp;$\Sigma^*$.

Remember that a reduction from $L_1$ to $L_2$ has to map "yes" instances of $L_1$ to "yes" instances of $L_2$ and "no" instances of $L_1$ to "no" instances of $L_2$.  Every input is a "no" instance of the language $\emptyset$ so, to reduce $\emptyset$ to any $L\neq \Sigma^*$, you just need to choose some word $w\notin L$ and your reduction maps everything to&nbsp;$w$.  Obviously, for $L=\Sigma^*$, this doesn't work because there's no $w$ you could use.

Similarly, $\Sigma^*$ can be reduced to any other language except&nbsp;$\emptyset$.