Retaining bolt

Gravity is unreliable.

This closely guarded fact is kept from the public by a cautious government for fear that it will cause panic. We naturally assume gravity will always be there and will always work, but this is not the case.

In certain circumstances gravity gets distracted and forgets the rules.

This is why, at the top of large (and therefore massive) hills you will always find the Retaining Bolt that keeps the whole thing screwed securely to Mother Earth.