People are often uncertain about whether there is any difference between can and may when these verbs are used to ask for or grant permission. For example, is one of these two sentences ‘more correct’ than the other? Can I ask you a few questions? May I ask you a few questions? There is a widespread view that using can to ask for permission is wrong and that it should only be used in expressions to do with ability or capability, e.g.: Can she swim? Can you speak Italian? 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. Resource: http://www.oxforddictionaries.com/words/can-or-may

'May' is more formal than 'can' when asking a question. May I use the bathroom? - formal Can I use the bathroom? - informal

*difference*