The Threat of an Elder Troll

If you must go out after dark, my child, keep to the familiar paths - don't stray.
An elder troll has made his home somewhere near town. The Big Strong Men and the Big Strong Women of town have not been able to find it's lair - it could be closer than you think!
They know it's there because of the footprints it left in the mud of your uncle's field. Big, deep footprints, with three big toes on each foot.
They know it's there because of the sheep that went missing that night. Not little lambs, mind you, full grown sheep. It swallowed them whole, you know.
You're safe inside, because it's an old, old troll and it wouldn't dare draw attention to itself. If the Big Strong Men and the Big Strong Women heard it crashing about houses, they would chase it and thump it and chain it and when the sun came up it would turn to stone.
But if you wander off in the dark, if you walk across the fields to visit a friend, or sneak out of the house..
Well, your Uncle has been protecting his sheep, so it's probably... getting ... HUNGRY!

-Granny

Summary

Knowledge of Trolloid Geriatric Petrification (to the extent that it is accurately understood) is preserved mainly though tales told to children, to keep them from wandering about after dark. The threat of an elder troll works perfectly for this purpose, since the sunlight-sensitivity makes it hard to prove the story false.
The details change in different locations. Sometimes the teller claims to know there's a troll by footprints, sometimes there's a distinct smell.