Learning is also often the result of making mistakes. Something new and difficult was introduced, tried, and needs practice. That learning can get messy. It requires more grit and focus than the easy lesson.
If you want to know what your child learned on any given day, you need to be prepared to notice a change and you need to be prepared to accept the failures of a student working through tough new concepts.
Change can lead students to question things they did not question before, use new language, change their opinion on a topic, etc. In some cases, this can prompt parents to have a, "Do you know what my child said when she got home yesterday? What are you teaching at that school?" response.
That is natural. Change is hard, but the evidence of change/learning provided by a student is most often an invitation to engage in a vulnerable conversation than a reason to shame a teacher (thanks Brene Brown!).
The path to learning is also littered with mistakes. I have frequently advised that the standard of satisfaction is not perfection. As a matter of fact, I was recently asked how I got to this point in my career. My answer was, "I made plenty of mistakes, but I learned from them." To err is human, but so is to learn. Opening up about learning requires a space safe enough to admit imperfection and ask for help. Simply put, a frank discussion about learning demands a growth environment (mindset).
So, do you really want an answer to "What did you learn today?" Are you prepared to engage in shame-free discussions about change? Are you prepared to nurture a growth environment that accepts imperfection?
Would you prefer your child simply said, "Nothing."