This time, I'm waiting for my sister, Bast to let me know when she's had the babies - she was going in for a Caesarean at 930 this morning, Sydney (Brisbane) time.
Anyways, I came across these - Temperature sensitive glass tiles.
I'm imagining them as a splashback in a kitchen, or tiles that line a shower recess (Like this)
They're priced at about $15 (US) per standard size 4" x 4" x 3/8" tile - a square foot will run you about $150 (US).
They're available through Inventables.