Beer forms a head because of a protein in the barley called albumin. When beer is poured, the dissolved carbon dioxide bubbles come out of solution and form bubbles. The albumin molecules in beer help to stabilize these bubbles and form a foam that rises to the top of the glass. Soda, on the other hand, does not contain albumin, so the bubbles in soda do not form a foam. Instead, they quickly dissipate, and the soda becomes flat.

In addition to albumin, the amount of carbon dioxide in a beverage also affects the formation of a head. Beer typically contains more carbon dioxide than soda, which helps to create a thicker, more stable head. The temperature of the beverage also plays a role, as warmer beverages tend to produce more bubbles.

The head of a beer can be an important part of the drinking experience. It can help to enhance the flavor and aroma of the beer, and it can also provide a visual cue as to the freshness of the beer. A good head will typically last for several minutes, while a thin or weak head may indicate that the beer is old or has been stored improperly.