Three of the following four numbers are alike in a certain way and one is different. Pick the number that is different from the rest.

121, 100, and 81 are perfect squares of odd numbers (11², 10²... wait — 100=10², but 121=11², 81=9², 16=4²). Actually 121, 81 are squares of odd numbers, but 16 and 100 are squares of even numbers. Among the four, 16 is the only one that is not a perfect square of a number that ends in 1 or 9, and more precisely 121=11², 81=9², 100=10² are all squares divisible patterns — 16 is the odd one as it is 2⁴ while others are squares of multiples.