Few games lend themselves to fan theories like Undertale does.
I only finished it a few weeks ago, but I've already been having fun tracking down and pondering the most popular theories about the game's world and characters.
Undertale spoilers follow!
One of Undertale's most memorable characters is the fearsome warrior Undyne, a eyepatch-wearing fish lady who chases the protagonist for the first chunk of the game. One of the central ideas behind Undertale's lore is that humans have a power called "Determination" that allows them to carry on after death; monsters lack determination, which is one of the reasons humans are so much more powerful than monsters.
Depending on how you choose to play, you can learn that despite being a monster, Undyne actually has some determination — enough that, if you kill her, she's able to resurrect herself a single time and continue fighting. It's not entirely clear how that happened, so fans have had a number of theories as to what the deal is.
This new video from Geek Remix's Mari takes an interesting tack: Maybe Undyne's determination is thanks to the love of her friend, the mad scientist Alphys?
The video lays out the full theory, but basically: One of the children who preceded the protagonist died in the grasses of the waterfall region, and Alphys discovered that the grass there retained some of the dead child's determination. (The grass is described as scientifically protected at one point, and that would explain why.)
As Alphys experimented on ways to inject determination into monsters — something you learn about if you explore her lab en route to the true pacifist ending — she realised that she could make special milkshakes out of the grass (you can find clues pointing to this in her lab) and began feeding the milkshakes to Undyne — with whom Alphys was secretly in love — to make her stronger. As Mari points out, this is an Extremely Anime way to demonstrate true love: Start sneaking a magical serum into the object of your affections, with the goal of secretly making her super strong. Ha.
Even if it's a stretch, it's an enjoyable one, and impressive as ever that people keep finding ways to fill in the blanks that Toby Fox left in the outer reaches of Undertale's lore. Nature abhors a vacuum, and so do fan theorists.