Tim Downie wrote:

What you really need a a jam jar with adjustable difficultly. Perhaps fit a
valve to the lid and suck it down to an adjustable level of vacuum. Then
you could clearly demonstrate that one technique would work better than
another (and teach your students something about the scientific process).

They include academics and research students in scientific disciplines,
so I don't think they'll be very impressed with my jam jars in that
regard!

Daniele