Each pollen grain was being unequally bombarded by the water molecules in motion. This causes the pollen grains to 'random walk' on the surface of the water.
The project can be viewed and downloaded by clicking on the link below the graphic.
This project is the 2-dimensional version of my 1-dimensional project, Feynman's Random Walk, that can be viewed at https://scratch.mit.edu/projects/11282377/.
The Scratch blog post can be seen by clicking on this link.A free PDF document that describes Brownian Motion in more detail and explains the coding can be had by sending an email request to email@example.com.