Aleister Crowley once told a joke which he said encapsulated everything you needed to know about magic. Because he was a posy wazzock his version was far more long-winded than mine, but this is the gist of it:
Two strangers are sitting together in a compartment on a train.
First man: "Excuse me, my dear fellow, but I couldn't help noticing that that box you are carrying has airholes in it. Does it contain an animal of some sort?"
Second man: "Yes, there's a mongoose inside."
First man: "Good heavens. Why are you carrying such an unusual animal?"
Second man: "Well, you see, it's my poor brother. He has serious problems with the drink, and has begin to see snakes everywhere, so I am taking him this mongoose so that he may be rid of them."
First man: "But, my dear fellow, these snakes that your brother sees aren't they imaginary snakes?"
Second man: "Ah yes, but this, you see, is an imaginary mongoose."