Here's why:

* Visible particles: Brownian motion is observable in particles large enough to be seen under a light microscope.

* Erratic movement: The particles move randomly and erratically, constantly changing direction. This is because they are being bombarded by the unseen molecules of the surrounding fluid (usually water or air).

* Molecular collision: The random collisions between the visible particles and the much smaller, invisible molecules of the fluid cause the erratic movement.

In summary: Brownian motion is the random movement of particles suspended in a fluid resulting from their collision with the fluid's molecules.