I was reluctant to post this since thermal physics has been so thoroughly abandoned by NGSS (HSPS) and AP Physics. And also because the apparatus that I use appears to be unavailable. And the ones I was able to get most recently are a bit fiddly to use.
But what the heck? Physics teachers teach across a spectrum and not everyone is beholden to NGSS and AP. And physics teachers are a resourceful bunch: There may be other ways to do this without the exact apparatus I'm using here.
The demonstration revolves around what I've been told is a Galilean thermoscope. A narrow-throated Florence flask is used here, as is a beaker, some food coloring, a twist-tie, a blow drier, a hot plate, and some water.
I like it enough to continue using in in my AP Physics 2 unit on thermal physics which is a pre-cursor to the unit on thermodynamics.
An instant Zoom poll using participant reactions is once again included.
Student Google Doc - Thermoscope
HTML Presentation - Thermoscope (includes animations and live-action video)
If you have a clever workaround to get past the dearth of what were once sold by Sargent-Welch as "air bulb thermometers," let me know in the comments.