Today I "borrowed" an idea from someone - I think it was Kathy - and microwaved a bar of soap. I used an old bar of soap that was almost gone, but hard to use due to a non-soap layer on part of the bar.
It started foaming up, but stopped when I opened a door to take a picture. (The soap was black with yellow and green, so it did not change colour.)
The kitchen still smells like the soap.