Think with your head, not with your eyes

"Think with your head, not with your hands" is a piece of advice I often give my students, especially when they are staring out. To this I may have to also add, "Think with your head, not with your eyes"
As part of some basic training exercises I gave them a number of positions where they had to spot the 'free' piece. It was a piece that could be captured for 'free' as normally there were no other pieces protecting it. Most of the positions were pretty easy (deliberately so), but I did throw the odd tricky position in. The diagram is an example of one of the tricky ones, and it caught most of my students out.
They all wanted to grab the queen on h5, missing that bishop on e2 was pinned. The correct answer was just as much a surprise to them as well, as they missed that the knight on c6 was also pinned! But what was the real surprise to me was when I included it the set of questions for the following week, most of them chose the wrong answer a second time.
What I think this means is that as a group they have moved to the level when they can 'see' moves, but are yet to progress to 'understanding' moves. Hopefully this will change over the next few months.