Most of the interesting-tasting or psychoactive chemicals that plants make are there to ward off being eaten or infected. Caffeine and mint oil are among the plant insecticides, all sorts of other things are toxins to vertebrates. By virtue of being megafauna we can tolerate amounts of toxic stuff that will kill smaller organisms by mixing them with other foods, and our particular biochemistry happens to be particularly strong against some (and weak against others, just try to eat hemlock). Stuff we can tolerate but still has effects on us (caffeine, capsaicin) can be interestingly psychoactive, stuff that doesn't hurt us can be interesting to taste (mint, cinnamon, garlic), and there's interesting correlations between spice use and parasite load in food (that could be confounded six ways to mars)...
Nicotine, too, is an insecticide.
Please reply in the comments with things you understood recently. The only condition is that they have to be useless in your daily life. For example, "I found this idea that defeats procrastination" doesn't count, because it sounds useful and you might be deluded about its truth. Whereas "I figured out how construction cranes are constructed" qualifies, because you aren't likely to use it and it will stay true tomorrow.
I'll start. Today I understood how Heyting algebras work as a model for intuitionistic logic. The main idea is that you represent sentences as shapes. So you might have two sentences A and B shown as two circles, then "A and B" is their intersection, "A or B" is their union, etc. But "A implies B" doesn't mean one circle lies inside the other, as you might think! Instead it's a shape too, consisting of all points that lie outside A or inside B (or both). There were some other details about closed and open sets, but these didn't cause a problem for me, while "A implies B" made me stumble for some reason. I probably won't use Heyting algebras for anything ever, but it was pretty fun to figure out.
Your turn!
PS: please don't feel pressured to post something super advanced. It's really, honestly okay to post basic things, like why a stream of tap water narrows as it falls, or why the sky is blue (though I don't claim to understand that one :-))