When water is heated in a narrow neck flask,why water surface falls in the begining?

I will use as an example, a volumetric flask. As you heat the large volume bulb at the bottom, the first thing that happens is that the gl container expands before the liquid does. Because of the larger container volume for a relatively unchanged liquid volume, the level of the liquid in the narrow neck initially drops. As the heat is eventually conducted through the gl to heat the liquid, only then will the liquid expand and move once again up the neck of the flask.