Nobody knew why, until our museum acquired the vase pictured below. It has as you can see a tube on the side, but also a pouring lip. Hence the conclusion that the tube is not there to pour, but to blow. Blow air into the cocoa drink to create foam, which was the part most liked by the Mayans.
The mystery of the "chocolateros" was resolved.But what is still not, is why these vases were used in the beginning and that at a later stage Mayans used to pour from one vessel into another to create foam.