Studies show that Americans now get twice as many calories from beverages as they did in teh 1960s. The sheer number and variety of caloric beverage options, the growing portion size of beverages and the way we "count" beverages as part of our diet (we don't) have combined to make what we drink a major culprit in poor nutrition and weight gain.
The body and mind register beverages differently from food which is why these calories can sneak up on people. According to the Center for Disease Control and Prevention (CDC), a drink's rapid passage through the mouth provides less time for signals to trigger the brain that you are eating. Compared to beverages, solid food takes time to chew and seems to provide a feeling of fullness, which signals the brain to stop eating. Thick liquids (like smoothies and shakes,) register fullness more than coffee and water.
Also, some research suggests a psuchological componet to our inattention to liquid calories. Many of us consider beverages a separate category that does not "count" in the same way as solid food, and studies found that there is rarely a reduction in food intake in response to beverage consumption.