Basic Pot Odds, how to explain to someone who doesn't agree?

[dblgutshot]

I am having a discussion with a friend about pot odds. It came up as I was saying, if the pot is laying me 15-1 on the river, and my hand is probably best 10% of the time I should definately call!

He agreed, but he says that 15:1 is not the odds you should be using. He argues that you must account for the money that YOU put in. If you remove that, then those are your true odds you should be using.

Now I understand that to be very very wrong! However I just cannot explain to him why this is not true. I tried to explain how the money is not yours once you put it in. Does anyone have a link to a good discussion of this? Or have a clear explaination?