Click on 'Rods', to choose rods and and drag them onto the squared background.
A rod can be rotated by $90^\circ$ by clicking any key whilst dragging.
You can change the background squares (smaller or larger) using the 'View' menu.
When you've done a lot of exploring you might like to try the something similar with numbers.
If you do, have a look at Number Spirals here.
The NRICH Project aims to enrich the mathematical experiences of all learners. To support this aim, members of the
NRICH team work in a wide range of capacities, including providing professional development for teachers wishing to
embed rich mathematical tasks into everyday classroom practice.