Evidently FedEx will not ship empty cans and containers if they have labels like rocket fuel or gravity.

FedEx guy: Is this really what this is? Rocket fuel?
Me [laughing, used to this reaction from being at the space-travel supply store]: Oh, no, no, no. Of course not. It’s… made-up. Kind of a joke. It’s actually full of sugar, which I just put in to give it some weight, but that’s all it is.
FedEx guy: You can’t ship this.
Me: But… it’s just sugar! What, uh, what if I empty it out? It doesn’t really need to have sugar in it.
FedEx guy: No. They would still x-ray it, and then you’d get a call when it was en route. I don’t think it would make it.
Me: Hmmm. I really need to get this out… but I guess I don’t have to ship the “Rocket Fuel” can. What about the rest?

via found on the web