The missing number in the sequence is 64, which goes between 27 and 125. The number sequence is of cubed numbers: 1 is 1 cubed, 8 is the number 2 cubed, 27 is the number 3 cubed and 64 is the number 4 cubed.
A cube number is a number that is multiplied by itself once, and then the result is multiplied by the original number once again. For example, 5 cubed would be 5 x 5 x 5. First, multiply 5 x 5, which is 25. When multiplied again, 25 x 5, the answer is 125. Cubed numbers can be visualized as blocks, such as a Rubik's cube, a 3 x 3 block that would contain 27 blocks if it could be dismantled.