Folks who want to make concrete some relations which may help student
understanding might consider the virtues of hot pennies.
Adding a hot penny at a time to a sample of cold water might well
show off a staircase of temperature versus number.
This is essentially your recommendation, John, but rendered concrete,
with all its possibility of scorching juvenile skin, it's true!
[What's so intuitive about an invisible pulse of electrical heating, anyway?]