Find the odd one from the given series:
$$4,16,36,64,100,121,196,256$$

Let's analyze the mathematical pattern of each number in the series:
* $4=2^2$ (Square of an even number)
* $16=4^2$ (Square of an even number)
* $36=6^2$ (Square of an even number)
* $64=8^2$ (Square of an even number)
* $100=10^2$ (Square of an even number)
* $121=11^2$ (Square of an odd number)
* $196=14^2$ (Square of an even number)
* $256=16^2$ (Square of an even number)

All the numbers in the series are squares of consecutive even numbers, except $121$, which is the square of an odd number ($11$). Thus, $121$ is the odd one out.