Charge an iPhone with an iPad?

This may seem like a weird question, but I was idly wondering is there a way--cable of some sort--that would charge an iPhone from an iPad. I mean, the iPad has a bigger battery, so it's theoretically possible to use some of that juice for the iPhone.

Maybe this question is only a thought experiment. I googled and found nothing.

Source: http://forums.macrumors.com/showthread.php?t=1286083&goto=newpost

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