The Elves Will Teach You

When children of many species do not behave, there's a common story told by many parents before bed. That the Elves will teach you.
WHAT exactly the elves will teach you, or which elves, or what the outcome might be, is quite varied. In the westernmost human kingdoms, it's commonly that the Elves will teach you 'a lesson' - or, in other words, punish you - for not going to bed, and the simplest version of this tale ends there. This version is more of a turn of phrase than a fully realised myth.
Elsewhere, however, the Elves might be said to teach you the secrets of the universe, mysteries of life and death, and the unknown powers of gods long dead... rendering you unable to sleep forever more, until you wither away and die, living out you remaining few days incapable of anything but the deepest philosophies. This is probably my favourite one, not least because it seems to imply that elves are capable of simply injecting the secrets of the universe into a child's brain, with a syringe or such.
Other less extreme examples include the Hydral Elves teaching you how to swim - I believe this to be a metaphor for bed wetting - or the Forest Elves teaching you how to eat your vegetables, which I have no explanation for as it seems to be far removed from a punishment any sensible parent would impart. The Uruk teach that the elves of their land will teach you the ways of war, and snatch peace forever from your heart - perhaps the most likely and the most scary, given the peaceful nature of the Uruk. The Gonatohud claim the elves will teach you to rely on others, and the Cubals teach that the Long Elves specifically (though it is unclear why) will teach you how to cheat at games of chance. And yes, the Elves in question find this highly offensive, especially considering the reputation Cubals get for being societal scroungers.
A slightly different variation of this tale, incidentally, is taught by Kho-sshurus to their youngest childen. These reptilian parents teach their children that the Planaar - not elves - will teach them how to feel bad, by instilling qualities like 'pride', 'regret' and 'jealousy'. This seems rather unusual, until you recall that the Kho-sshurus are highly emotional creatures whose life focuses on how events make them feel, while the Planaar usually learn to withold their emotions and feelings due to how other species often view them as celestials, elementals and demons.