This comment was posted to reddit on Dec 30, 2020 at 11:25 am and was deleted within 5 minutes.

One way to look at its is filling in different regions. If you have an object n and need to fill its first region you have n possibilities you have n^{1} combinations. Then for the second region you would have n possibilities for each region, so n*n=n ^{2.} Then for the third region, n*n*n=n

Using the example above in the water molecule there is only one oxygen region and there are only 16 isotopes to choose from so 16^{1.} However there are two hydrogen regions each can be filled by one of 7 different isotopes thus there are 7*7 or 7 ^{2} combinations. Thus in total there are 16*7

Sorry about the formatting im on mobile.