Our father told us that the best way to test whether a steaming cup of anything was cool enough to drink was to just drink it. It wasn't until the fifth time we were in the hospital that we discovered that the man wasn't our father. If we had these heat-sensitive On/Off mugs, which change from the Off position when cool to the On position when hot, we'd still have our original tongues. However, as Joel from BBG points out, the heat-sensitive plastic lining has a tendency to wash off—leading to a scenario where a practical joker could paint the mug black and write OFF on the side, then pour in a cup of boiling hot coffee. [Charles and Marie via Technabob via Boing Boing Gadgets]
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BAH! too hot - throws coffee at assistant.