Superfluid Helium
Although Core requirements generally end freshman year, there are many options that require general classes from other departments. An oft-taken class in the breadth requirements for majors is Phys 2c - which is stat mech. In 2c, they recently did a demonstration for a superfluid, which was actually liquid Helium at temperatures below 2.5 K.
First of all, the Helium had to be cooled to that temperature, as it starts at more than twice the ideal temperature. Using evaporative cooling, and by creating a vacuum in the vessel where the liquid helium is, the helium can reach temperatures as low as 1 K. As it cools, you can see the bubbles of warmer helium rise to the top of the liquid. However, as soon as the helium hits a certain temperature (during the demonstration it was around 2.1 K), the bubbles stop appearing. It’s because at that point the liquid helium started behaving as in superfluid. The whole of the liquid is the same temperature, so there are no pockets of warmer helium elsewhere in the helium to bubble up.
The liquid helium also had a couple of other cool properties (which I don’t totally understand - I’m not actually taking the class). For one, when the liquid helium level was brought below a beaker (and the beaker was filled with liquid helium of the same temperature), the helium would drip over the sides of the beaker, even when it was not overflowing. It actually dripped until the level of the beaker was half a centimeter below the top of the beaker. The other property involved making a fountain of liquid helium. However, as there was no pump to propel the liquid to make a fountain, it was a rather interesting property of the ideal liquid.
