Using an extremely high voltage source (several million volts worth), it is possible to inject a a very large charge of electrons into a block of acrylic. Since acrylic is an excellent insulator, it will take many hours for the electrons to bleed off. However, a sharp blow with a grounded metallic point will (1) stimulate them to move and (2) provide them with an exit point to which to flow.

This is shown in the video below. As the electrons gather up into ever large streams, they heat up the acrylic and leave their mark as “frozen lightening”.

This is also known as a Lichtenberg figure.

[rr: this was pre-recorded, since I am currently on holiday]