Grass seeds are placed onto a projection slide with conductive material. An abundance of grass seeds are placed on the projection slide randomly. Two electrodes (which aren't connected) are placed on the projection slide, and a Van de Graaff Generator is used to charge them. This creates a potential difference between them. The grass seeds feel this potential difference, and align with the electric field between the electrodes. This creates a great visual of electric field lines.
To see it in action: https://www.youtube.com/watch?v=gsC3mMcWOkM&feature=youtu.be