Show that the application
$$\begin{array} { | c c c }
\left( \mathcal { M } _ { n } ( \mathbb { R } ) \right) ^ { 2 } & \rightarrow & \mathbb { R } \\
( A , B ) & \mapsto & \operatorname { tr } \left( A ^ { \top } B \right)
\end{array}$$
is an inner product on $\mathcal { M } _ { n } ( \mathbb { R } )$.