A tricky problem...

There are 10 machines that fill bottles of soda. One of the machines is either putting one ounce (of mass) too much or one ounce too little in the bottles that it produces. You have a digital scale. In one weighing figure out which of the 10 machines is off. The puzzle does not involve any tricks. The bottles are opaque etc. Note you can assume that you know what the correct weight is, say 10 oz.

I don't know the answer, but am thinking about it A LOT. Posted at Marginal Revolution and they'll post the answer tomorrow.