Yes, I know this one has four feet. Sorry! Photo by Robert Lawton
Nowadays a trivet is anything that's used to stop a hot pot burning a table (though it has to be a bit more substantial than a mat or a coaster). It might be metal, wooden, or even made of silicon.
Here are two:
A chopping board, for instance, is not a trivet unless it's being used as one. Then it is. Then later it'll disappear and become a chopping board again.
The philosophers among you may find this interesting.
If you never enter a kitchen, and are never served from a very hot serving dish, then you might be able to spot a trivet in the mirror because you yourself might be as right as a trivet (I hope you are). Right as a trivet is a rather old-fashioned expression meaning in perfect health.
I can find two explanation of the origin of this odd phrase. One is probably wrong, but is much too gloriously eccentric not to share.
Spot the Frippet: a trivet. The Old English form of this word was thrifēte. The Latin tripēs means three-footed. The word trivet was in use as trevet by the early 1400s.
There's a story that trivet in the expression right as a trivet is a garbled form of Truefit, the name of a maker of very fine wigs. But the expression steady as a trivet is old, and the expression, sadly, probably came from there.
My guess is that right as a trivet implied as level as a trivet (right as in right-angle) because, well, you try cooking a fried egg on a sloping trivet and you'll find out why.
I was a Girl Guide, so I have experience.