What is the correct sentence when asking for something like while flying asking for a glass of water :
May I have a glass of water
Can I have a glass of water
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The Cambridge Dictionary website ("English Grammar Today") ranges three possible modal constructions in this order of increasing politeness.
Can I ask you a question? (Informal)
Could I use your phone? (More formal/polite)
May I use your phone? (Even more formal/polite
The Oxford Dictionary also says that "Can I" is possible, although less formal:
But the 'permission' use of can is not in fact incorrect in standard English. The only difference between the two verbs is that one is more polite than the other. In informal contexts it’s perfectly acceptable to use can; in formal situations it would be better to use may.
So, if you aim for the most polite construction, you might say
May I have a glass of water, please?
It never hurts to add "please" to make the request more polite.