This seems dangerous. I know he's wearing gloves, but my understanding is that the real danger of working with mercury like this is that it evaporates rather quickly, turning into an inhalant.

When breathed in, it can be quite toxic.http://www.health.state.mn.us/divs/eh/hazardous/topics/mercury.html

It sounds like he is using a breathing mask too, from the sound of his breathing. Narration is added later, obviously, as the experiments are done in reverse.

It'd be nice if he used a real sponge (like one you'd use to wash a car with), rather than a piece of packing material.

OK, so Why?

Why won't it enter the sponge? why won't the liquid enter the negative spaces in the sponge?

I do know there are some materials that don't absorb water - I have some dish towels (bought at the dollar store) that won't absorb water, absolutely useless. But I think that's different as I doubt they would absorb anything.

So why not Mercury? it's definitely more dense than water. Are the molecules larger than the H2O Molecule? I don't think it is, and even if it was, surly they are not larger than the holes in the sponge.

I'd like to see this tried with different types of sponges.

I have often said that the sailors of the future will sail on sponge ships upon seas of mercury.

Next up: Mercury - Will it Blend?

stunt sponge after the cut.

I suggest that it's most importantly to do with the surface tension of mercury. When the mercury covers any of the sponge's holes it forms a new little surface where the strong surface tension matters. You can tell this by looking at any transparent container of mercury such as old barometers and such. The meniscus bends down at the edge rather than the equivalent container of water where it bends up a little. If you pour out a bunch of mercury on a flat surface it will spread out so it's about half a centimeter deep iirc, i used to play a bit with some as a kid. (sometimes in a vac chamber, but not always!)

Here's a thought though. Unfortunately he had parts of the sponge poking out (possibly because of the limits of his container/supply of mercury) and so air could refill the holes in the sponge and allow it to retake its preferred shape. But if it was entirely submerged in the mercury and squeezed, which would be stronger? The springiness of the sponge to want to return to its shape sucking mercury in, or the surface tension of the mercury and the sponge just stays squashed. And i know that packing material is more springy than a bath sponge (or a natural sponge).

Adhesion. Water is polar, its why you get surface tension and I think that's why it sticks to most surfaces. Mercury adheres to itself pretty good but for different reasons. You'll notice it runs off surfaces in a similar way water runs of a hydrophobic surface - it doesn't streak because it doesn't stick.

Now, perhaps if it were an aluminium sponge...

Ok, so here is a very good point.
Because of strength and density, squeezing the air out of the sponge, while submerged under the mercury..., where is that air supposed to go?

Displacement. for the mercury to fill the sponge, the air must be pushed out and displace mercury while submerged, Can air, displace the Mercury in this instance? that's another interesting factor here.

Surface tension sounds plausible, but with the pressure of squeezing the sponge, I would think that tension would break even a little.

From Reddit:
"This can be explained through the principles of cohesion and adhesion. Water has strong cohesion to itself and strong adhesion to the sponge. Mercury has strong cohesion to itself but weak adhesion to the sponge.

Cohesion arises from attraction between something and itself. Cohesion is strong in water due to the large amount of hydrogen bonding between water molecules, causing water to stick to itself. Cohesion is strong in liquid mercury due to something called metallic bonding between metal cations and delocalized electrons in the liquid metallic liquid, causing the mercury to stick to itself.

Adhesion arises from attraction between something and something else. Since water molecules are polar, they can hydrogen bond with the polyurethane sponge. Mercury's metallic bonding, however, does not interact with the non-metallic sponge. Mercury will, however, adsorb to other metals such as gold or silver and form an amalgam.

Basic mercury clean up kits usually contain metal sulfides, which can react with the metal, disrupting metallic bonding, and permitting other forms of intermolecular forces to facilitate clean up."

I was going to write nearly the same thing. It needs to be submerged.

My physics lab prof in college said:

"If you ever see a question about 'why does water do x while {insert other liquid here} does y', the answer is always 'because it's polar'".

Expanding when frozen? Polar.
Strong surface tension, adhesion and cohesion? Polar.
Complex molecules dissolving in water? Because the water AND the dissolving molecules are polar. Non-polar things *won't* dissolve in water, for the reverse reason.
...
And I guess this can be thrown on the list also, although just saying "polar" misses some of the details/nuance that Sagemind covered.

Next up: Does salt dissolve in mercury?