Sometimes, I'm amazed by some of the questions I get from GCSE and A level students. Sometimes these questions demonstrate a really high level of thinking about the subject and a deep understanding.
And then some of them are really fundamental that should have been told to them years ago.
for example, I was once balancing an equation to a group of year 11s when one of them put up their hand and asked "So what's the difference between the big numbers and the little numbers?"
"Er, the big numbers mean that the substance isn't bonded together and the little numbers mean that the atoms are bonded together, so O2 (can't do subscript) means that the oxygens are joined and 2O means that there are 2 oxygens that aren't."
When I asked if anyone had told these year 11s this, there was a chorus of "No!"
However, year 11s probably would have seen equations since year 7 and they would have had to balance them in year 10 at the latest. They had been spending four years smiling and nodding at their teachers who had been writing equations in front of them, oblivious to what was going on in the students' heads.
I got reminded of this recently when a student asked me how recrystalisation works.
"Well, it's because hot liquids dissolve more than cold ones."
It turns out that no one had told her this simple fact, either, and, what is more, it was not even on the exam specification, which is usually very thorough about these things.
One of the troubles with teaching is that I can forget that something used to be difficult, or, worse, there are several things that I never found difficult that most people can't do and, when you start out, you have no idea what mistakes they could make. It always helps to find someone who nos nothing about what you will teach and explain it to them, so you can understand the questions they will ask and how to answer them, because you I may have spent my whole life assuming that something I know is common knowledge.
I also try to learn things myself to remind myself what a state of ignorance is like and what I would do in that situation just so I can see where the gaps are. And what i have realised is that nothing goes without saying.