This morning I went to the bakery and ordered two croissants to go. The barista placed each one in an individual wax paper bag (for the prices they charge, she should have placed them in gift-wrapped boxes). Then she asked me, "Would you like a bag for the bags?" I said yes, and she produced a real bag of heavier paper and put the two croissants in.
"I feel so funny every time I say 'Would you like a bag for the bags?" she said. "It guess it's tautology."
"I think it's more like a paradox," I suggested.
It occured to me as I left the store that what she was really grappling with was metonymy, a grammatical (actually, rhetorical) concept James Thurber immortalized in his essay about his English teacher, Miss Groby ("Here Lies Miss Groby," from The Thurber Carnival.) Metonymy is when you say "The iced tea is great; can I get you a glass?" What you really mean, of course, is can you get the person some tea, but the container (glass), stands in for the thing contained (tea).
You could read about metonymy and its subcategory synecdoche at World Wide Words. But you'd get much more out of reading The Thurber Carnival.