Water droplets form on the outside of a glass of cold soda due to a process called condensation. The cold temperature of the glass cools the air immediately surrounding it. Since warm air contains water vapor and can hold more moisture than cold air, when the warm, humid air touches the cold glass, it cools down quickly. As the air cools, it loses its capacity to hold water vapor, causing the excess water vapor to condense into liquid water droplets on the glass surface. This is similar to how dew forms on grass in the morning. The water droplets are not coming from inside the glass but from the moisture present in the surrounding air. The phenomenon is more pronounced on warm, humid days because the air contains more water vapor to begin with.