An inverted test tube partially filled with air floats in aplastic water-filled soft-dri...

An inverted test tube partially filled with air floats in aplastic water-filled soft-drink bottle as shown in Video V2.7 and Fig. P2.78. The amount of air in the tube has been adjusted so that it just floats. The bottle cap is securely fastened. A slight squeezing of the plastic bottle will cause the test tube to sink to the bottom of the bottle. Explain this phenomenon.