Actually that does not demonstrate the greenhouse effect!
From the PDF at the link: “This activity mirrors how a greenhouse works, but it’s not exactly the same as the greenhouse effect that is taking place in the Earth’s atmosphere.”
The effect of the actual greenhouses is due to the glass preventing the air dissipating its heat due to convection.
The poorly-named “greenhouse effect” doesn’t actually explain how greenhouses work, it has to do with the so-called “back-radiation”.
Do you have a link to an experiment that demonstrates the actual effect in action? It should be easy to find one if the effect is real.