[DoctorBeaver (OP)]

Water is. I don't think mercury is. So what does "wet" mean?

[RD]

The verb to wet : to coat with a film of liquid.
The ability of a liquid to wet a surface is inversely proportional to its surface tension.
Water has a high surface tension, so most liquids are "wetter" than water, (e.g. pure alcohol).
Mercury, like water, has an unusually high surface tension.

[lyner]

Adhesion vs cohesive forces, I would suggest. Because of it's high specific heat capacity, once water  has wetted you it also manages to feel very cold. Other wet liquids - like light oils - stick but don't feel as cold.
I don't think there's an official definition.
Mercury wets copper braid very well btw.

[RD]

Mercury reacts with (dissolves) copper ...

Metals such as tin, silver, and copper will form compounds with the mercury.

http://www.madsci.org/posts/archives/2003-04/1049252616.Gb.r.html

[DoctorBeaver (OP)]

hmmm, interesting

[lyner]

Water dissolves lots of things - eventually.
I guess dissolving is the ultimate attraction.

[lightarrow]

Mercury reacts with (dissolves) copper ...

Metals such as tin, silver, and copper will form compounds with the mercury.

http://www.madsci.org/posts/archives/2003-04/1049252616.Gb.r.html

Ok, but, at room temperature, a mercury drop will first wet a copper surface, then, after xyz days/weeks will have (perhaps) completely dissolved in it. To increase reaction speed you would use fine copper powder. By the way, using fine Cu powder is a way (invented by me and never experimented []) of removing very little Hg drops from pavement after a possible crash of an Hg thermometre.
Maybe gold is even more efficient, but it depends on your budget...  []

[lyner]

Never drop mercury on the floor of an aeroplane - it eats its way through to the outside as the dissolved aluminium oxidises at the mercury air surface.

[Kryptid]

What about when you get down to the molecular level? What if you had a single molecule of water held to a single molecule of sugar by intermolecular forces? Is the molecule of sugar "wet"? If not, then how large does a group of molecules have to be before can be wet?

[Monox D. I-Fly]

The verb to wet : to coat with a film of liquid.

Going with this definition, can someone wet something with lava?

[evan_au]

Perhaps one aspect that did not appear in the above discussion is the type of forces between molecules & atoms.
- Water is a "Polar" molecule, with the Oxygen end being more negative than the Hydrogen end. Water interacts more strongly with Polar molecules (eg water) than it does with non-Polar molecules. So water will not wet things like wax, which is pretty much non-Polar.
- Mercury is a metal, which achieves much of its surface tension by sharing electrons. This interaction also works when in contact with copper or gold, but not when in contact with glass (an insulator).

The meniscus of mercury in glass is the opposite direction of the meniscus of water in glass.
See: https://en.wikipedia.org/wiki/Meniscus_(liquid)

[Bored chemist]

This might help
https://en.wikipedia.org/wiki/Contact_angle