Which number is the odd one out: 36, 81, 136, 169?

Explanation

1. 01

   $36 = 6^2$, $81 = 9^2$ and $169 = 13^2$ are perfect squares.

2. 02

   136 lies between $11^2 = 121$ and $12^2 = 144$, so it is not a perfect square.