How can use adsense on youtube???

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I am a regular visitor of youtube, its a great site, i know that people upload their video on youtube to share whole world. but recently when i was watching the you tube video "ads from google" also appearing on the screen. I know its from google adsense ( I have an adsense account too), But i dont know how it comes on the vide, is any special feature on youtube or adsense to show ads between video. Please if any one using that suggest me to place ads on my video..