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What have in common Venetian gondola, celtic stone, time arrow and anti-matter? They are all slightly asymmetric.
The Matter (negative electrons) and anti-matter (positive electrons, i.e. positrons) seem to be perfectly symmetric.
The first surprise came in 1957 from somewhat ephemeral (i.e. difficult to detect) particles called neutrinos. It turned out, that they turn-around only in one direction.
If the charge of neutrino is opposite, it rotates in opposite direction. It seems that the electrical charge (C) is related to the rotation directions (we call it parity) in a way that the combination C-P is maintained.
Next surprise came in 1964, from exotic, but heavy and rather long lived particles coming to Earth due to cosmic radiation - kaons. It was observed that kaons do not obey to C-P conservation. C-P is "more left that right".
But the most recent experiments [1,2] show that if we consider together charge, parity and time, the combination C-P-T is conserved. In other words, it must be the time which is asymmetric, compensating the C-P asymmetry.
That's clear: we get older every day!
The C-P asymmetry is small, something like one part every ten million. But this is sufficient to make time going in one direction and matter being million times more abundant in Universe than anti-matter.
In the same way the "celtic stone" or Venetian gondola are slightly asymmetric but they turn (the stone) or get rowed (the gondola) always in one direction.
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[1] A. Angelpoulos et al. "A
determination of thr CPT violation paramater Re(d) from semileptonic decay
if strangeness-tagged neutral kaons", Phys. Lett. B444 52 (1998)
[2] F. Abe et al. "MEasuerements of the CP-Violation Parameter sin (2b)
in Bd-J/K Decays", Phys.Rev. Lett. 81 5513 (1998)