HTML element id changes at runtime

See more:

ASP.NET

HTML

Ok i have an ASP.NET web form, i've always wondered why at runtime the ID's change of my html elements if they're placed inside of a asp:control

e.g

<asp:Content ID="Content2" ContentPlaceHolderID="ContentBody"  runat="server">
<img id="produt_image">

at runtime becomes

<img id="ContentBody_product_image">

why does it do this, and does it affect naming / calling that img box from the code behind, would i ref it using product_image or the newly runtime id?