Potions, in the world of Harry Potter, are fascinating. No spells are cast over the cauldrons. The magic seems to come from preparing the right ingredients in the right way, adding them at the right times and temperatures, and stirring them in the correct patterns. It seems like something a Muggle could do!
Some potions are more interesting than others. Here's a list of a few potions I think are particularly worthy of note.
Ageing Potion
Does just what it says on the bottle: makes you older. The more of it you drink, the older you get—physically.
Fred Weasley and Lee Jordan used this potion in an attempt to get across Dumbledore's age line to put their names in the Tri-Wizard Cup. They failed, thus establishing that whoever put Harry's name in either did it before the age line went up, or was over 18.
What interests me about this potion is that it doesn't seem to be temporary, suggesting that if you want to be (physically) young again either you can't, or there's actually a youth potion out there somewhere.
Amortentia
This is the potion that gave us Voldemort!
A kind of magical roofie, this potion will force whoever drinks it to love you against their will. Voldemort's mother used this to enslave (and rape) the Muggle she fancied, keeping him drugged for years. When the potion wore off, he fled, terrified and angry, leaving her and the unborn Voldemort (Tom Riddle) behind.
Developing solution
A potion that makes photographs move! I really like the idea of this. According to Colin Creevey, it works on otherwise perfectly normal Muggle negatives. Now, that's cool.
Felix Felicis
How in the heck is my instinctive response to this luck potion. A potion which, when drunk, alters the world around the drinker in the drinker's favor? How?? You expect a potion to have its effect on the person who drinks it, not on the things and people who didn't. Yet when Harry drinks this potion, what he wants to do becomes possible, with other people's actions "luckily" changing to make his plans work.
The closest I can come to understanding how this potion might function is guessing that it gives the drinker a subconscious knowledge of their immediate future—a kind of low-level, short-lived seer ability.
Polyjuice Potion
A shape-change potion that seems to work on the same principle as a voodoo doll: add the hair or nails of another human to this potion, and whoever drinks it will physically metamorphose into the victim. Perfect for identity theft (and it was used that way for nearly an entire book)!
The catch to this potion is that the shape-change is temporary—in order to stay in shape, as it were, you have to brew and drink this potion regularly.
What interests me is that Polyjuice Potion can't be used for non-human transformations. So who counts as non-human? We all know that adding cat hair to this potion results in tragedy. But what would happen if you added the hair or nails of, say, Remus Lupin? I wonder.
What are some of your favorite potions?