>>And the youngest one just had an assignment for her chemistry class to do some chemical process at home, and make a video of it, so she donned the yellow rubber gloves and, under mom's supervision, made another batch. When I put the video together, I named the file "soap opus.avi". She actually didn't know that "opera" was plural of "opus", so I had to slip that bit of information in an en passant manner, during an unrelated talk. And then the pun worked, when she saw the filename again :).
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>I didn't know opus was the plural of opera, either. Thanks!

And you shouldn't, because it isn't. It's the other way around: opus - the work, opera, the works.