>>From my daughter's class - someone just corrected someone, that the proper plural of "childhood" is "childrenhoods". Corollary: mousepad - micepads.
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>That's like saying that collective humanity is humanities, which we all know is a doddle of a degree course

Another corollary: if it's "mousepads", then it's "micepad" - after the same logic in which "tom hanks" is singular, and "toms hank" is plural. That's quite simple, because it's a simplest form of sentence: subject + verb.