Stability of ortho- and para-hydrogen
From "Statistical Mechanics: A Concise Introduction for Chemists" by Benjamin Widom (pp. 35-36):


  For the diatom of ordinary hydrogen ($\ce{{}^{1}_{1} H}$), where each
  nucleus is just a proton of spin quantum number 1/2, there are three
  possible symmetric states of the pair of spins (nuclear spin triplet)
  and only one possible antisymmetric nuclear spin state (spin
  singlet). [...]
  
  At high temperatures, which generally means $T \gg
&gt; \theta_{\mathrm{rot}}$ (where $\theta_{\mathrm{rot}}$ is the
  characteristic rotational temperature) and so for $\ce{H2}$ means $T$
  well above 88 K, the three ortho nuclear-spin states and one para
  state are equally populated, so equilibrium hydrogen at high
  temperatures is a mixture of ortho- and para-hydrogen in the ratio
  3:1. In pure hydrogen the magnetic interactions between molecules that
  could cause transitions between nuclear spin states are so weak that
  such hydrogen may be cooled to arbitrarily low temperatures with the
  ortho:para ratio remaining 3:1. That is normal hydrogen. Paramagnetic
  impurities such as $\ce{O2}$, or a catalyst such as $\ce{Pt}$ on the
  surface of which $\ce{H2}$ molecules may dissociate, allowing the
  atoms to recombine with their nuclear spins in a different relative
  orientation, catalyze the ortho-para conversion. Cooled in the
  presence of such a paramagnetic impurity or such a catalyst, then, the
  hydrogen may have its ortho:para ratio continually adjust to that
  which would correspond to thermal equilibrium at the given low
  temperature. [...]
  
  Cooled in such a way, then, hydrogen becomes pure para-$\ce{H2}$ at
  low temperatures, in contrast to "normal" hydrogen, which remains a
  mixture of ortho and para in the ratio 3:1. If the hydrogen is cooled
  in the presence of a catalyst that catalyzes the ortho-para conversion
  and thus allows it to become pure para-$\ce{H2}$ at low T, and if the
  catalyst is then removed and the hydrogen warmed back up to 298 K,
  say, the result is a sample of pure para-$\ce{H2}$ at normal
  laboratory temperatures. Pure ortho-$\ce{H2}$ cannot be prepared in
  this way.