That bizarre milk scene from The Last Jedi has a basis in dairy science

Warning: Spoilers ahead for Star Wars Episode VIII: The Last Jedi.
There’s a strange scene in The Last Jedi as Rey is following Luke Skywalker around his hermitage on the planet Ahch-To. Luke has apparently gotten into the habit of milking the planet’s local creatures — the remarkably relaxed thala-sirens that spend their time reclining against the rocks. He walks up to one of the creatures and squeezes green milk from its prominent udders. Then he drinks it, soaking his beard