Let $G$ be a compact Lie group. A torus in $G$ is a subgroup $T \leq G$ isomorphic to $(S^1)^n$ for some $n \geq 0$, where we set $(S^1)^0 = \{*\}$ which is the trivial torus.
A standard theorem asserts the existence of a maximal torus $T$ in $G$, can it be trivial when $G$ has positive dimension? Why not?