If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
Notes for future OT posters:
1. Please add the 'open_thread' tag.
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3. Open Threads should be posted in Discussion, and not Main.
4. Open Threads should start on Monday, and end on Sunday.
Would "because it's insufficiently challenging" be at least as good an explanation?
Only if had a control where I tried to make things harder without using interleaved practice. I haven't done that, I have less reason to suspect that simply making learning harder would make you learn better.