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In Boy's homework the other day, one of the activities was a game involving 4-digit additional.  Needed to play was a "number cube". 

A number cube.  Was that, I wondered, like a die (singular of dice)?  Or was it something else, some new aid for teaching math?  It threw me into a bit of a tizzy for a brief moment because I was pretty sure we didn't have a number cube in the house and Boy wouldn't be able to finish his homework.  But as I read on in the instructions for the game, I came to understand that, no, a number cube was a die. 

What could possibly be wrong with the word "die"? Did the word suddenly become politically incorrect when I wasn't paying attention?  What reason is there for changing the name of that little piece of plastic?  Am I missing something?