There's a beer on Emily's table, before Norm brings her one.
When Diane balances the mug full of "beer" on her head and walks across the bar without spilling it, it is obvious that the contents of the mug is not actually a liquid. There are no bubbles in the beer and the foam and liquid in the mug do not slosh around or move as Diane puts the mug on the top of her head and walks away with it.