Can or May??

May: used when asking for permission.

Student: 'May I go to the restroom?'
-not, 'can I go to the restroom?' of course you can, but your teacher has to give you the permission to.

Can: possibility.
Is it possible to do something.

John: 'Can I lift that heavy box?"
-not, 'May I lift that heavy box?' (In this case, John is asking if he has the permission to lift the box, while in the first question, he is wondering if he has the ability to lift the box.