Menu and Content not working.

I'm new to WPF. I've just been trying to put a menu bar and a grid object in a window, but I always get the exception that I am trying to set a content property more than once. I can't figure out how to put a menu along with other elements in the window.

Code:

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<Window x:Class="WpfApplication1.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="Eliptical Button">
 
        <Menu IsMainMenu="True">
            <MenuItem Header="File">
                <MenuItem Header="New"/>
                <Separator/>
                <MenuItem Header="Open"/>
                <MenuItem Header="Save"/>
                <Separator/>
                <MenuItem Header="Save As..."/>
                <Separator/>
                <MenuItem Header="Exit"/>
            </MenuItem>
            <MenuItem Header="Edit"/>
        </Menu>
   
    <Grid>
        <Button/>
    </Grid>
</Window>

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Can someone please help me to get this right? Thanks.

you must put the menu and the grid with the button (or just the button) in a single main grid so the main window will have only one child