We've all been there.  You're sitting around the dinner table and you ask your child, ​"What did you learn in school today?" 
Their response is usually something like this:

Every day I randomly chose a student or two and ask, "Whatcha gonna tell yo momma?"  They have to tell me one thing they learned. 
Hopefully this will remind all of my students of something they learned during the day.
If they don't answer with something more than a lost look, keep asking.  Eventually you should get an answer.