My children were a little apprehensive at first to try this activity. I encouraged them to be gentle with the mirror and ensured that I was close by to supervise - I knew my daughter would understand the fragile nature of the mirror but wasn't so sure my three-year-old son would.
The children instantly noticed their reflections on the smooth working surface.
It was hard to tell where the real structure ended and the reflected one began...
In addition to a neat mini-lesson on reflective surfaces, this activity allowed me to discuss the differences between 2D and 3D shapes and introduce some terminology (e.g., cube, triangular prism). It was also interesting to see my children problem solve how to stack the blocks so they did not fall as their towers got taller. You'll notice the 'sidewalks' my son created that joined the structures together. He had a wonderful imaginative story for why the inhabitants of the structures needed to be able to visit each other.
For a great article on how block play encourages numeracy, visit Block Play, Math and Literacy.