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Given the following post, respond as if you're commenting on Reddit:
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ELI5:bose-einstein condensate

What is it?

How is it made?

What practical applications does it have?

What's it's history?
gpt
A Bose-Einstein condensate is a very special phase of matter which certain elements and compounds are thought to be able to enter.  One which is known to be able to enter it is helium-4, which we call superfluid (slightly different but closely related) when it does so.  The origin comes from statistical and quantum mechanics and can be a little complicated but I'll try to give a breakdown.  

**Fermions vs. bosons:** one way to categorise particles is by their spin.  Spin is an intrinsic property of particles (like charge or mass), and goes in units of 1/2.  Fermions are particles which have half-spin mutliples (e.g. 1/2 or 3/2), whilst bosons are particles which have integer spin multiples (e.g. 0 or 1).  Spin belongs to elementary particles like electrons with spin 1/2, or photons with spin 1, but also to composite particles like atoms.  As an example, helium 4 is composed of four protons, four neutrons and four electrons, each with spin 1/2.  But these spins pair up and align against each other (e.g. two electrons pair up, one has spin +1/2 and one has spin -1/2, so the pair has a total spin of zero).  This means that even though helium-4 is made of fermions, it is itself a boson.  

**Quantum states:** quantum mechanics tells us that particles live in something called a state.  The state has a defined energy (among other qualities).  The lowest energy state is called the *ground state*.  

**Statistical distributions:** fermions and bosons follow two different statistical distributions (Fermi-Dirac and Bose-Einstein respectively).  These tell us how ensembles of these particles behave.  The long and short of it is that fermions can't share the same state, whilst bosons can.  

With these things in mind, lets consider an ensemble of bosons (e.g. helium-4).  What happens when it cool it right down towards absolute zero?  Well at some point (for helium-2, about 2.5 kelvin), all the particles drop into the lowest energy state, the ground state, from all the states above.  We now have a state of matter in its lowest possible energy configuration.  That's why Bose-Einstein condensates from.  It's tough to explain without a bit of quantum mechanics.  You make it, as said, by cooling the right kinds of particles down to the transition temperature for it to happen.   

In terms of history, Bose-Einstein condensates were predicted in 1924-25, then superfluid helium-4 was experimentally made about 12 years later.  It finds applications as a quantum solvent, letting scientists study particles as though were in a gas; in high-precision gyroscopes, and in cooling devices when things have to be cooled to extremely low temperatures.  Superfluid helium-4 was also used to slow down light, effectively trapping it, in an experiment in 1999.