What it shows:
The redshifted spectrum of galaxies and quasars is due to an expanding universe and can be expressed as the ratio of the scale factor of the present Universe to that of the Universe when the light was emitted. You can think of this as the light being s-t-r-e-t-c-h-e-d as the Universe expands so it arrives with a longer wavelength.
How it works:
A 50cm × 10cm strip of dental dam with a wave drawn on it, attached at one end to a post and the other end free to pull. A wooden dowel at the pulling end ensures an even stretch.
figure 1. elastic light sheet
Useful demo to explain cosmological redshift without using the word "doppler". And Margaret Geller likes it, so that's good enough for us.