So glad you asked, this is a really interesting feature of #neutrinos!

Neutrinos are weird for a number of reasons. They're fermions (particles of matter), but unlike all the rest of the fermions they're much lighter (in mass) than all the other fermions and we're not sure why. https://twitter.com/TheDesignClass/status/1285845593064120320

In fact, for a long time we thought neutrinos were massless (ie had zero mass). But in 1957 a physicist called Bruno Pontecorvo suggested that maybe neutrinos actually had mass after all. It took many years to validate this, and it came with some really cool consequences!

Neutrinos come in three flavours: electron, muon, and tau. But each neutrino flavour is made up of three different mass states (which we call 1,2,3). It's kinda like making 3 different cocktails from the same 3 ingredients, just mixed in different quantities.

Mathematically, you can describe it by the superposition (adding up) of waves. In the pic below there are only two mass states for simplicity, but you can see how different ways of adding ν1 and ν2 give you different shapes, which represent ν_electron and ν_muon

NOW HERE'S THE REALLY COOL THING! As neutrinos travel through space, what's really happening is that their ν1, ν2, and ν3 components are travelling through space. And it turns out that they don't all travel at the same speed! So the superposition (ie flavour) changes over time!

So that's how you can start out with a muon neutrino, and then end up with an electron neutrino (or tau neutrino) some time later!

For more general information and background, @StartsWithABang has a really nice article here https://www.forbes.com/sites/startswithabang/2020/07/14/how-massive-neutrinos-broke-the-standard-model/#7e19c2805380

And the cool animation, with a more technical article is from @iplus_elusives

http://www.elusives.eu/outreach/entry/ceaseless-transformation-three-neutrinos

Hope you enjoy!