Full Idea
The critical formula that distinguishes S5 from all others is: ◊p → □◊p.
Clarification
S5 is the best known system of modal logic
Gist of Idea
◊p → □◊p is the hallmark of S5
Source
Rod Girle (Modal Logics and Philosophy [2000], 3.3)
Book Reference
Girle,Rod: 'Modal Logics and Philosophy' [Acumen 2000], p.34
A Reaction
If it is possible that it is raining, then it is necessary that it is possible that it is raining. But if it is possible in this world, how can that possibility be necessary in all possible worlds?