Find the odd number out:

Let us check the divisibility properties or common factors of the given numbers:
* $24=24\times 1$
* $72=24\times 3$
* $168=24\times 7$

All options except $36$ are completely divisible by $24$ (multiples of $24$). Alternatively, $36$ is a perfect square ($6^2=36$), whereas the others are not. Therefore, $36$ is the odd number out.