Okay, I promise this is the last time I'll do a "radical freedom" joke. Although when you think about it no promise that I make today can actually determine my future actions, on account of...well, you know.

And yes, to answer your question, regrets are made out of water.

Thales is probably the oldest Western philosopher that we have record of, so in that sense he "invented philosophy" (although I'm sure there were people before him that asked the same kinds of questions, whose work was lost). His theory, basically, was that all that exists is composed of a single substance: water. Today we know that water is composed of other things, but the idea that everything is only different modalities of a single thing is still alive, whether that thing is a certain kind of boson, or quantum fields, or whatever. How Thales would respond to questions about whether abstract concepts like "infinity", or subject experiences like "sadness" were also simply made of water we don't know, but those kind of questions are probably what motivated Plato to have a more complex theory of existence beyond simply matter, in his theory of forms.
Anaximander and Anaximenes were students of Thales who worked on similar problems, who also believed in a kind of monist system, i.e. that reality was made of only a single thing.

A true Übermensch spends Christmas miserable and alone, as everyone knows.

Nietzsche's doctrine of Amor Fati, to love one's fate, meant that a chief task in our lives is to affirm life and love one's destiny. He believed life to be deterministic, but we could not give in to nihilism or despair, and should love what must unfold. His own life was filled with hardships and disappoints, to say the least. He had constant problems with his health, multiple rejected marriage proposals, and as the comic shows he suffered a mental collapse and ended life in the care of his sister. She edited his final work, The Will to Power to suit her far right politics, and Nietzsche's ideas were used by the Nazis to justify their dominating the Earth. Nietzsche, of course, would have hated this. He wrote against both anti-semitism and nationalism in his life. The challenge, of course, for Nietzsche would then be what he thought was the challenge for all of us: to nonetheless love his fate.

Also, existentialism is when, like...you are too cool to care that smoking kills you, because we are all gonna die anyway and stuff.

Postmodernism, in philosophy, is a term originally coined by Jean-François Lyotard in The Postmodern Condition. He defined it as the condition of society where people were generally skeptical of "metanarratives". A metanarrative, for Lyotard, is a grand narrative of all humanity which we are collectively working towards in some way. For example, that science will break down all truths to reductionist physics, or that a dialectical method will resolve the contradictions in society to a point where humanity can live in an ideal way. Lyotard was using the term denoscriptively, and in a negative sense. In other words, he did not advocate for postmodernism, but was merely describing a collapse in the belief in modernity. The term is often also closely associated with "post structuralism", and Jacques Derrida, and a sort of "irrationality". Like Lyotard, Derrida may have believed that what Enlightenment thinkers thought of as "Reason" wasn't as simple and straightforward as was previously believed, and that the term itself had subjective, historical, and cultural meaning imposed upon it by context in which it was used, so a narrative that we can easily move forward to being "governed by reason" was rooted in the dominate culture imposing their version of "reason". He certainly didn't believe this sort of analysis was "irrationality", and even less so advocated for abandoning reason itself, but advocated for a more careful analysis of what "reason" is. He did this, of course, through what he considered rational discourse.
Unlike these historical and philosophical definitions, the term "postmodernism" is used by many far right pundits as a sort of boogeyman. What they mostly mean by it is some sort of Marxism (who was an very modernism thinker, in that he thought the material dialectic was the Grand Narrative of history), or more vaguely, anyone who is at all to the left of them is an irrational "postmodernist". These people are generally advocate for a return to "reason" or "rationality", which somehow always equates to more concentrated power in the hands of people exactly like themselves. They ignore, of course, that no one in history has advocated for being against reason, only for understanding what reason is, and how it operates in our culture today. Of course, they don't care about any of these philosophical debates, they just want to reverse the tide of feminism or whatever. Well, tough luck for them, because it isn't going to happen.

Also, existentialism is when, like...you are too cool to care that smoking kills you, because we are all gonna die anyway and stuff.