Why do this problem?
is very useful for deepening children's understanding of reflection, as well as helping them to focus on visualising things.
This activity is probably best approached with a group of pupils so that whilst they can see the challenge they can talk freely with you in order to understand clearly what they need to be thinking about and imagining in their heads.
How can you decide what goes where?
Tell me about your answer.
It can easily be extended by adding to the number of blocks used by one of the group when the others are trying to produce the reflection.
Cubes and safety mirrors are obviously required. The other thing that pupils are likely to need is time!